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Thoughts on the werewolf incident What precisely was going on and what did Sirius think he was achieving?
[cut] Harry got up. [cut] He walked across the Hall and opened the door into the chamber. [cut] 'Surprise!' Mrs Weasley said excitedly, as Harry smiled broadly, and walked over to them. 'Thought we'd come and watch you, Harry!' She bent down and kissed him on the cheek. [GoF ch. #31; p. 534/535]
Harry could tell that, when James stood up, they would be within an inch of each other in height. [OotP ch. #28; p. 565]
Narcissa Malfoy strolled out from behind the clothes rack. [cut] 'Really?' said Harry, taking a step forwards and gazing into the smoothly arrogant face that, for all its pallor, still resembled her sister's. He was as tall as she was now. [HBP ch. #06; p. 110]
Once the painful transformation was complete, he was more than six feet tall [DH ch. #12; p. 197]
James was exactly the same height as Harry. [DH ch. #34; p. 560]
When Snape said nothing, Narcissa [cut] staggered to Snape and seized the front of his robes. Her face close to his [cut] Snape caught hold of her wrists and removed her clutching hands. Looking down into her tearstained face [HBP ch. #02; p. 39]
[cut] Black was a tall, full-grown man. [PoA ch. #17; p. 249]
'I'll get to the point, then,' said Sirius, standing up. He was rather taller than Snape [OotP ch. #24; p. 459]
The day was fine and breezy, and neither of them felt like staying indoors, so they walked past the Three Broomsticks and climbed a slope to visit the Shrieking Shack, the most haunted dwelling in Britain. It stood a little way above the rest of the village, and even in daylight was slightly creepy, with its boarded windows and dank overgrown garden. 'Even the Hogwarts ghosts avoid it,' said Ron, as they leaned on the fence, looking up at it. 'I asked Nearly Headless Nick ... he says he's heard a very rough crowd live here. No one can get in. Fred and George tried, obviously, but all the entrances are sealed shut ...' [cut] Someone was climbing towards the house from the other side of the hill. PoA ch. #14; p. 205/206]
They covered the distance to the trunk in seconds, but before they had reached the gap in the roots, Crookshanks had slid into it with a flick of his bottle-brush tail. Harry went next; he crawled forwards, headfirst, and slid down an earthy slope to the bottom of a very low tunnel. Crookshanks was a little way along, his eyes flashing in the light from Harry's wand. Seconds later, Hermione slithered down beside him. 'Where's Ron?' she whispered in a terrified voice. 'This way,' said Harry, setting off, bent-backed, after Crookshanks. 'Where does this tunnel come out?' Hermione asked breathlessly from behind him. 'I don't know ... It's marked on the Marauder's Map but Fred and George said no one's ever got into it. It goes off the edge of the map, but it looked like it ends up in Hogsmeade ...' They moved as fast as they could, bent almost double; ahead of them, Crookshanks's tail bobbed in and out of view. On and on went the passage; it felt at least as long as the one to Honeydukes ... All Harry could think of was Ron, and what the enormous dog might be doing to him ... he was drawing breath in sharp, painful gasps, running at a crouch ... And then the tunnel began to rise; moments later it twisted, and Crookshanks had gone. Instead, Harry could see a patch of dim light through a small opening. It was a room, a very disordered, dusty room. [PoA ch. #17; p. 247]
As though invisible strings were tied to Snape's wrists, neck and knees, he was pulled into a standing position, head still lolling unpleasantly, like a grotesque puppet. He hung a few inches above the ground, his limp feet dangling. [PoA ch. #19; p. 276]
Getting back into the tunnel was difficult. Lupin, Pettigrew and Ron had to turn sideways to manage it; Lupin still had Pettigrew covered with his wand. Harry could see them edging awkwardly along the tunnel in single file. Crookshanks was still in the lead. Harry went right after Black, who was still making Snape drift along ahead of them; he kept bumping his lolling head on the low ceiling. [PoA ch. #20; p. 277]
'What – live with you?' he said, accidentally cracking his head on a bit of rock protruding from the ceiling. [PoA ch. #20; p. 277/278]
Sirius turned right around to look at him; Snape's head was scraping the ceiling [PoA ch. #20; p. 278]
Harry wriggled into the earthy passage hidden in the tree’s roots. It was a much tighter squeeze than it had been the last time they had entered it. The tunnel was low-ceilinged: they had had to double up to move through it nearly four years previously, now there was nothing for it but to crawl. Harry went first, his wand illuminated, expecting at any moment to meet barriers, but none came. They moved in silence, Harry’s gaze fixed upon the swinging beam of the wand held in his fist. At last the tunnel began to slope upwards and Harry saw a sliver of light ahead. [cut] He groped behind him and she forced the bundle of slippery cloth into his free hand. With difficulty he dragged it over himself [cut] and continued on his hands and knees [cut] And then he heard voices coming from the room directly ahead of them, only slightly muffled by the fact that the opening at the end of the tunnel had been blocked up by what looked like an old crate. Hardly daring to breathe, Harry edged right up to the opening and peered through a tiny gap left between crate and wall. [DH ch. #32; p. 523/524]
As quietly as he could, he pulled himself up into the room. [DH ch. #32; p. 528]
'I speak now, Harry Potter, directly to you. You have permitted your friends to die for you rather than face me yourself. I shall wait for one hour in the Forbidden Forest. If, at the end of that hour, you have not come to me, have not given yourself up, then battle recommences. This time, I shall enter the fray myself, Harry Potter, and I shall find you, and I shall punish every last man, woman and child who has tried to conceal you from me. One hour.' [DH ch. #33; p. 529]
Yaxley looked down at his watch. 'Time's nearly up. Potter's had his hour. He's not coming.' [DH ch. #34; p. 562]
Before we consider what actually happened in the werewolf "prank", we need to know what the layout of the tunnel from the Whomping Willow to the Shrieking Shack is. In truth, the Doylist explanation is that the shape of the tunnel is whatever suited Rowling at the time and probably changed according to her whim: but to analyse canon we need a Watsonian explanation which fits the text.
First off, the size of the tunnel is measured in people – mostly Harry and Snape. In summer 1995, at the time of the third task of the Triwizard Tournament and a year after Harry first went down the tunnel under the Whomping Willow, not-quite-fifteen-year-old Harry is still so much shorter than Mrs Weasley, a short woman, that she has to bend down to kiss him, even though he is definitely standing. He is not more than 4'10", tops. In summer 1996, just-sixteen-year-old Harry is as tall as Narcissa Malfoy, who is tall for a woman, so maybe 5'8", so he has grown ten inches in just over a year. A couple of months beforehand he sees his sixteen-year-old father as an inch taller than himself. In summer 1997, just-seventeen-year-old Harry is 6ft or less: probably noticeably less, since he really notices when he is Polyjuiced into a body over 6ft tall. In May 1998 he is the same height as adult James: but James was apparently not tall, so that probably means about 5'10".
Sirius is described as tall, but not as very tall. Snape is enough shorter than him that there is a visible difference when they stand face to face, but Harry doesn't seem to have noticed the difference before and Sirius is only "rather" taller, so the difference in their heights is probably only two or three inches. Meanwhile, Snape is enough taller than Narcissa, who is tall for a woman, that he looks down at her as she clutches at his chest, although she may not be standing straight. John Nettleship, on whom Snape is based, was 5'8" but we can't make Snape that short without also making Sirius too short to be called "tall". We can reasonably assume therefore that Narcissa is about 5'8", Snape 5'10" and Sirius 6ft to 6'1".
The tunnel is described as "very low", at least at the Willow end, when Harry and Hermione go down it in PoA. It is low enough that Harry, when he is 4'10" or less, goes down it "bent almost double": and four years later the now about 5'10" Harry has to go down it on all fours. It is also narrow enough that people go down it in single-file. Overall it's probably about 3ft wide and 4ft high, maybe 4'6".
For reasons explained in the Map of Hogwarts section, we know that it's about three quarters of a mile from the Willow to the Shrieking Shack, even if the tunnel runs straight, so there is plenty of room for it to not be uniform: but the description of Harry "running at a crouch" until almost the end in PoA sounds like it is low the whole way.
At the end the tunnel rises – which is what you'd expect given that the school grounds broadly slope down towards Hogsmeade, and then the Shack is on a hill. The tunnel must climb quite a long way up, from however far below the ground it is and then the height of the hill.
At the top of the slope it twists before you get to the opening into the Shack, which is close enough that in DH some light gets past the twist. Or, of course, Rowling clean forgot about the twist when she wrote DH, but I'm trying to accommodate all the information we are given, and in any case the twist is mentioned in PoA which is also where we get most of our information about the "prank". The entrance to the Shack is small and opens either through the floor or very low down on a wall, because Harry climbs up through it to access the Shack, and it can be blocked by a crate.
When they leave the Shack in PoA Snape is upright and a few inches off the floor, which would require a clearance of 6ft even allowing for the fact that his head scrapes the ceiling. There is also no mention of anyone crouching, yet Sirius is meant to be taller than DH Harry and in DH Harry had to go on hands and knees. However, we are only really told about the start of the tunnel on this return journey. It may be that there is only enough clearance to move Snape upright while they are descending the slope, where the vertical axis is at a diagonal to the floor, and once they get to the bottom the adults have to go on all fours and move Snape prone. But if the tunnel only has a bore of about 4ft all the way then to achieve a head-room of 6ft it would have to be inclined at far too steep an angle to climb, so it must be higher at the slope end, as well as tilted. Or perhaps the level area "round the twist" where the entrance to the Shack is is bigger than it sounds, more like a 6ft-long landing on a staircase, and it was only there that Snape and Sirius were upright.
Either that, or the tunnel actually changes size and shape whimsically, in-universe as well as in the story. It's not impossible, if it behaves like part of the castle.
That still leaves the issue of speed. Snape must have got down the tunnel heading towards the Shack very fast: but perhaps he can already fly, and so was able to lie prone and then streak down it like a bird. But there's still an issue with how fast Harry and Hermione manage to get back along the tunnel in DH. It takes Harry a few minutes over an hour to get back along a three-quarters-of-a-mile tunnel which he supposedly has to negotiate on all fours, cross the grounds, move an unknown distance through the castle horizontally, ascend eight storeys plus tower to the Head's office, watch Snape's memories, climb back down, cross the grounds again and make his way to Aragog's nest which is at least one and a third miles into the Forest ("as the crow flies" – probably more "as hounds ran"). Perhaps the process of downloading Pensieve memories into the brain is near-instant, even though it reads as if it took at least twenty minutes to view Snape's memories.
They transformed ... Peter, as the smallest, could slip beneath the Willow's attacking branches and touch the knot that freezes it. They would then slip down the tunnel and join me. Under their influence, I became less dangerous.' [PoA ch. #18; p. 260]
A cloud shifted. There were suddenly dim shadows on the ground. Their party was bathed in moonlight. [cut] Harry could see Lupin's silhouette. He had gone rigid. Then his limbs began to shake. 'Oh, my –' Hermione gasped. 'He didn't take his potion tonight! He's not safe!' 'Run,' Sirius whispered. 'Run! Now!' [cut] There was a terrible snarling noise. Lupin's head was lengthening. So was his body. His shoulders were hunching. Hair was sprouting visibly on his face and hands, which were curling into clawed paws. [PoA ch. #20; p. 278/279]
Next question: when does Remus transform? In PoA, he doesn't change until the actual moonlight hits him. But that can't be normal, otherwise all any werewolf would have to do to avoid transformation is to stay in a room with the blinds drawn. Also, in PoA he doesn't transform in the Shack, only after he has been exposed to full moonlight outside; yet it is clear that as a boy he used to transform inside the Shack.
My working theory, which I've used in my fanfics, is that the fact that Remus has been taking Wolfsbane all week, even though he didn't take his final dose, has weakened his response to the moon, so that he now only transforms when the moonlight hits him, whereas without Wolfsbane he would just transform at moonrise. When he is on Wolfsbane he could indeed choose not to transform at all just by avoiding moonlight, but he doesn't do so because if he did, his next month's transformation would be worse.
An alternative theory is that the fact that in PoA he transforms when the moonlight hits him is sheer coincidence. He transforms when the moon becomes full. That need not happen exactly at moonrise: it can happen when the moon is already in the sky – or indeed when the moon is not visible, during the day. Note that at full moon the moonrise coincides closely with the sunset, which is not so at other times of the month.
In the latter case, Remus might remain a werewolf until the moon ceased to be full, which means he would probably remain in wolf form until well into the next day, because whether the moon is full or not depends on whether it is on the far side of the Earth from the sun or not: not on whether people at a given point on the planet can currently see it or not. But that depends on how finicky you are about defining when the moon is full. It could be said to be only full for a split second as it transitions from waxing to waning, or at the other extreme, for about three and a half days surrounding that split second.
Another important point is that we know he transformed in the Shack, because that was where the howls came from. There is no reference to howls coming from under the earth, and it seems unlikely that Remus would have been allowed to roam the tunnel in wolf form, because it would have been fairly easy for him to barge out past the Willow before it could hit him. That suggests that Remus was actually confined to the Shack during his transformations, which also fits with the statement that the other three slipped down the tunnel to join him. You can't get in at the Shack end, until Voldemort opened it up in DH, and presumably can't get out either, since it was intended to contain Remus. They must emerge from the Willow end before they run off to explore. Yet the other three don't just wait near the Willow for Remus to emerge: they have to go in and get him.
'The truth.' Dumbledore sighed. 'It is a beautiful and terrible thing, and should therefore be treated with great caution. However, I shall answer your questions unless I have a very good reason not to, in which case I beg you'll forgive me. I shall not, of course, lie.' [PS ch. #19; p. 216]
'Well, they did rather detest each other. Not unlike yourself and Mr Malfoy. And then, your father did something Snape could never forgive.' 'What?' 'He saved his life.' 'What?' 'Yes ...' said Dumbledore dreamily. 'Funny, the way people's minds work, isn't it? Professor Snape couldn't bear being in your father's debt ... I do believe he worked so hard to protect you this year because he felt that would make him and your father quits. Then he could go back to hating your father's memory in peace ...' [PS ch. #19; p. 217]
'I would hate you to run away with a false idea of your father, Potter,' he said, a terrible grin twisting his face. 'Have you been imagining some act of glorious heroism? Then let me correct you – your saintly father and his friends played a highly amusing joke on me that would have resulted in my death if your father hadn't got cold feet at the last moment. There was nothing brave about what he did. He was saving his own skin as much as mine. Had their joke succeeded, he would have been expelled from Hogwarts.' PoA ch. #14; p. 210]
'And they didn't desert me at all. Instead, they did something for me that would make my transformations not only bearable, but the best times of my life. They became Animagi.' [cut] Finally, in our fifth year, they managed it. They could each turn into a different animal at will.' [cut] 'They couldn't keep me company as humans, so they kept me company as animals,' said Lupin. 'A werewolf is only a danger to people. They sneaked out of the castle every month under James's Invisibility Cloak. They transformed ... Peter, as the smallest, could slip beneath the Willow's attacking branches and touch the knot that freezes it. They would then slip down the tunnel and join me. Under their influence, I became less dangerous. My body was still wolfish, but my mind seemed to become less so while I was with them.' [cut] Soon we were leaving the Shrieking Shack and roaming the school grounds and the village by night. Sirius and James transformed into such large animals, they were able to keep a werewolf in check. I doubt whether any Hogwarts students ever found out more about the Hogwarts grounds and Hogsmeade than we did ... And that's how we came to write the Marauder's Map, and sign it with our nicknames. Sirius is Padfoot. Peter is Wormtail. James was Prongs.' [cut] 'That was still really dangerous! Running around in the dark with a werewolf! What if you'd given the others the slip, and bitten somebody?' 'A thought that still haunts me,' said Lupin heavily. 'And there were near misses, many of them. We laughed about them afterwards. We were young, thoughtless – carried away with our own cleverness. [PoA ch. #18; p. 259/260]
'Professor Snape was at school with us. He fought very hard against my appointment to the Defence Against the Dark Arts job. He has been telling Dumbledore all year that I am not to be trusted. He has his reasons ... you see, Sirius here played a trick on him which nearly killed him, a trick which involved me –' Black made a derisive noise. 'It served him right,' he sneered. 'Sneaking around, trying to find out what we were up to ... hoping he could get us expelled ...' 'Severus was very interested in where I went every month.' Lupin told Harry, Ron and Hermione. 'We were in the same year, you know, and we – er – didn't like each other very much. He especially disliked James. Jealous, I think, of James's talent on the Quidditch pitch ... anyway, Snape had seen me crossing the grounds with Madam Pomfrey one evening as she led me towards the Whomping Willow to transform. Sirius thought it would be – er – amusing, to tell Snape all he had to do was prod the knot on the tree-trunk with a long stick, and he'd be able to get in after me. Well, of course, Snape tried it – if he'd got as far as this house, he'd have met a fully grown werewolf – but your father, who'd heard what Sirius had done, went after Snape and pulled him back, at great risk to his life ... Snape glimpsed me, though, at the end of the tunnel. He was forbidden to tell anybody by Dumbledore, but from that time on he knew what I was ...' [PoA ch. #18; p. 261]
BANG! Thin, snakelike cords burst from the end of Snape's wand and twisted themselves around Lupin's mouth, wrists and ankles; he over-balanced and fell to the floor, unable to move. With a roar of rage, Black started towards Snape, but Snape pointed his wand straight between Black's eyes. [cut] 'The joke's on you again, Severus,' Black snarled. 'As long as this boy brings his rat up to the castle –' he jerked his head at Ron, '-- I'll come quietly ...' 'Come on, all of you,' he said. He clicked his fingers, and the ends of the cords that bound Lupin flew to his hands. 'I'll drag the werewolf. [PoA ch. #19; p. 263–265]
With every ounce of strength he could muster, Harry raised his head a few inches and saw an animal amidst the light, galloping away across the lake. Eyes blurred with sweat, Harry tried to make out what it was ... it was bright as a unicorn. Fighting to stay conscious, Harry watched it canter to a halt as it reached the opposite shore. For a moment, Harry saw, by its brightness, somebody welcoming it back ... raising his hand to pat it ... someone who looked strangely familiar ... but it couldn't be ... [PoA ch. #20; p. 282]
'Sirius Black showed he was capable of murder at the age of sixteen,' he breathed. 'You haven't forgotten that, Headmaster? You haven't forgotten that he once tried to kill me?' 'My memory is as good as it ever was, Severus,' said Dumbledore quietly. [PoA ch. #21; p. 286/287]
'It takes years to become an Animagus, and then you have to register yourself and everything,' said Hermione vaguely, now squinting down the index of Weird Wizarding Dilemmas and Their Solutions. 'Professor McGonagall told us, remember ... you've got to register yourself with the Improper Use of Magic Office ... what animal you become, and your markings, so you can't abuse it ...' [GoF ch. #26; p. 423/424]
A couple of weeks after his dream of Rookwood, Harry was to be found, yet again, kneeling on the floor of Snape's office, trying to clear his head. [cut] 'Did you see anything unusual on your way down here, Potter?' Harry shook his head. Somewhere above them, the woman screamed again. Snape strode to his office door, his wand still held at the ready, and swept out of sight. Harry hesitated for a moment, then followed. The screams were indeed coming from the Entrance Hall; they grew louder as Harry ran towards the stone steps leading up from the dungeons. [OotP ch. #26; p. 520–524] 'They sometimes kill,' said Ron, who looked unusually grave now. 'I've heard of it happening when the werewolf gets carried away.' [HBP ch. #22; p. 442]
'... he's a – a bit of a mess, that's all. Greyback attacked him. Madam Pomfrey says he won't – won't look the same any more ...' Ginny's voice trembled a little. 'We don't really know what the after-effects will be – I mean, Greyback being a werewolf, but not transformed at the time.' [cut] 'No charm will work on these,' said Madam Pomfrey. 'I've tried everything I know, but there is no cure for werewolf bites.' 'But he wasn't bitten at the full moon,' said Ron, who was gazing down into his brother's face as though he could somehow force him to mend just by staring. 'Greyback hadn't transformed, so surely Bill won't be a – a real –?' [cut] 'No, I don't think that Bill will be a true werewolf,' said Lupin, 'but that does not mean that there won't be some contamination. Those are cursed wounds. They are unlikely ever to heal fully, and – and Bill might have some wolfish characteristics from now on.' [HBP ch. #29; p. 571/572]
She had just set some knives to work, chopping up steaks for Griphook and Bill, who had preferred his meat bloody ever since he had been attacked by Greyback. While the knives sliced away behind her, her somewhat irritable expression softened. [DH ch. #25; p. 412]
'I still say I saw a stag Patronus!' shouted the first Death Eater. 'Stag?' roared the barman. 'It's a goat, idiot!' [DH ch. #28; 450]
'NO!' shrieked Hermione, and with a deafening blast from her wand Fenrir Greyback was thrown backwards from the feebly stirring body of Lavender Brown. He hit the marble banisters and struggled to return to his feet. Then, with a bright white flash and a crack, a crystal ball fell on the top of his head and he crumpled to the ground and did not move. [DH ch. #32; 519]
'They sneak out at night. There's something weird about that Lupin. Where does he keep going?' 'He's ill,' said Lily. 'They say he's ill –' 'Every month at the full moon?' said Snape. 'I know your theory,' said Lily, and she sounded cold. 'Why are you so obsessed with them, anyway? Why do you care what they're doing at night?' 'I'm just trying to show you they're not as wonderful as everyone seems to think they are.' The intensity of his gaze made her blush. 'They don't use Dark Magic, though.' She dropped her voice. 'And you're being really ungrateful. I heard what happened the other night. You went sneaking down that tunnel by the Whomping Willow and James Potter saved you from whatever's down there –' Snape's whole face contorted and he spluttered, 'Saved? Saved? You think he was playing the hero? He was saving his neck and his friends' too! You're not going to – I won't let you –' 'Let me? Let me?' Lily's bright green eyes were slits. Snape backtracked at once. 'I didn't mean – I just don't want to see you made a fool of – he fancies you, James Potter fancies you!' The words seemed wrenched from him against his will. 'And he's not ... Everyone thinks ... Big Quidditch hero –' Snape's bitterness and dislike were rendering him incoherent, and Lily's eyebrows were travelling further and further up her forehead. 'I know James Potter's an arrogant toerag,' she said, cutting across Snape. [DH ch. #33; p. 540/541]
our common room lies behind a concealed entrance down in the dungeons. As you'll see, its windows look out into the depths of the Hogwarts lake. We often see the giant squid swooshing by – and sometimes more interesting creatures. We like to feel that our hangout has the aura of a mysterious, underwater shipwreck. [Welcome to Slytherin prefect's letter, Pottermore]
JKR: Lupin was very fond of Lily, we'll put it like that, but I wouldn't want anyone to run around thinking that he competed with James for her. She was a popular girl, and that is relevant. But I think you've seen that already. She was a bit of a catch. [Spartz-Anelli interview, 16th July 2005]
What do we know about the incident itself? For reasons explained here, it is virtually certain that the Marauders were already bullying Severus before the werewolf incident, and we know that Remus played at least some part in the bullying, and mocked him for having a big nose.
We know that Severus knew (and Remus, at least, knew that he knew) that Remus was being taken down the tunnel by Madame Pomfrey. Therefore, he knew that whatever was happening with Remus was sanctioned by the school staff. It's unlikely that he thought that by finding out that Remus was a werewolf he could get Remus into trouble per se, or not without getting Dumbledore and the whole school, probably including himself, into trouble. But at least, if he can confirm that Remus is a werewolf, the next time Remus jeers at his big nose he can say "At least I don't have a tail".
What else? It's clear in the courtyard scene that Severus has repeatedly told Lily that he thinks that Remus is a werewolf, even though his talking about it bored and irritated her. Rowling has said that Remus was "very fond of Lily", even though there's no indication of this in the books, where the only thing he ever says about Lily is "She started going out with [James] in seventh year". But if indeed there was any kind of attraction there Severus might have felt jealous – if, indeed, he actually fancied Lily himself and didn't just love her the way Harry loves Hermione, which is never really established – and wanted to show her that Remus was deceiving her. If so, he at least wasn't persecuting and torturing Remus out of jealousy the way James did him. But also, he may fear that Remus is a danger to Lily, especially if he thinks Remus fancies her. We see from what happens with Greyback and Bill that wounds caused by a werewolf, even when in human form, are cursed and can cause permanent scarring and behavioural changes. Even an accidental scratch from Remus might be dangerous, so Severus might well be motivated by concern for Lily: but now he can't tell her that his suspicions have been confirmed, because Dumbledore has sworn him to secrecy. Lily interprets "I won't let you –" as him being controlling but as usual she talks actross him and doesn't let him speak, so it was probably going to be "I won't let you sleepwalk into danger", or similar, since now he knows Remus is a werewolf. And he's right to be worried, because associating with the Marauders led to her death, albeit the danger was from Peter, not Remus.
He is concerned about what the Marauders may be doing when they sneak out at night, and thinks that finding out what it is will prove to Lily that they are dodgy. That sounds like they are already Animagi, and letting Remus out so they can map the school. He is concerned that James fancies Lily: again, it's not clear how much is jealousy, how much is hoping that his childhood best friend isn't going to pair up with the bully who torments him, and how much is concern that James is a criminal and a threat to her. If, as is implied, this is after James became an Animagus then he is a criminal: an unregistered Animagus who is letting a werewolf out to endanger the neighbourhood and then laughing about it, and who is working on a surveillance device which allows him to spy on a school full of children as young as eleven.
So, Snape sees Madame Pomfrey taking Remus to the Willow, and in some way Sirius tricks him into going down there. Note that there is absolutely no reason, as commonly assumed in fanfiction, to think that Severus broke curfew to do so (or that he was punished for this or for anything else after the incident). If Poppy Pomfrey is taking Remus all the way to the Shack she needs to allow about half an hour before moonrise, if moonrise is the trigger, to take him down a three-quarters-of-a-mile-long tunnel which, unless they are both exceptionally short or taking a Shrinking Solution, they have to go down at least bent double and possibly on all fours. We don't know exactly when the incident happened, except that it was after Sirius's sixteenth birthday on 2nd November 1975, and before the SWM scene in June 1976, but there are reasons to think it was at least several months before the SWM scene, so probably not later in the year than March and probably earlier. In December 1975 in the Highlands, in a spot we know is also surrounded by mountains, full-moonrise was at about 3pm and so Madame Pomfrey would be taking Remus down the tunnel at around 2:30pm, not long after lunch, and throughout winter it would be long before curfew.
At the same time, if Severus knows that the Marauders have been sneaking about at night he's probably been sneaking about himself, unless he spotted them during an Astronomy class. The Slytherin common room looks out on – or rather into – the lake, not the grounds, and it's unlikely their dorms are enough higher to have a view of any part of the grounds, except perhaps the near-side shores of the lake. The fact that he was working out spells in the margin of a sixth-year Potions textbook during fifth year suggests that Slughorn might have given him the use of a lab., but the Potions department too seems to be below ground: adult Snape's classroom and office certainly are.
Sirius tricked Severus into going down the tunnel. We do not know whether he allowed Severus to think he had accidentally overheard the information, or whether he told him outright. If he told him, perhaps it was framed as a dare: dares are sacrosant, so Severus would not think that even Sirius would cheat on a dare and set him up to die.
Note that those who say that it was all Sev's fault for trusting Sirius are essentially saying that Sirius was so blatantly insane that it should have been obvious that he would try to kill a classmate. Also, as explained here, there are a lot of tunnels from the castle to Hogsmeade and you can normally get out and into the village at the other end. Severus would have no reason to expect that the tunnel from the Willow ended in a dead end: he would probably expect that Remus was being taken out at the far end, thence to a separate building.
Then what happened? Severus went down the tunnnel, bent double or on all fours depending on how tall he was at that age, unless he flew by broom, and he "glimpsed [Remus] at the end of the tunnel". Remus also says "if he'd got as far as this house, he'd have met a fully grown werewolf", and that James pulled Snape back "at great risk to his life".
However you slice those statements, they are problematic, and suggest that perhaps Remus's memory of what happened while he was transformed may be a bit confused. If he means that Severus wouldn't have encountered him unless he reached the Shack, that confirms that he was normally locked in but raises questions about why Severus would be in danger in the first place. Surely they wouldn't expect Severus to be stupid enough to actually enter the Shack, as opposed to just looking through the entrance, and James would be in no danger at all so long as Severus didn't enter the Shack and Remus wasn't loose. And clearly Severus did not enter the Shack, because Remus says "if" he had got as far as the Shack he would have met the werewolf: so in what way was James in any danger?
It also raises big questions about how Severus could have "glimpsed [Remus] at the end of the tunnel" if he didn't reach the Shack. From the tunnel proper you go up a long steep slope, and since the tunnel has a low ceiling you wouldn't be able to see more than a few feet up the slope until you were standing right at the bottom of it. Then at the top the passage goes round some kind of corner (literally round the twist) and then there is a small entrance into the floor of the Shack, or low down on a wall. It really shouldn't have been possible to see somebody inside the Shack – even assuming that whatever barrier kept Remus confined was transparent – unless you first ascended the slope, rounded the twist and stood in front of or under the opening, and that fits poorly with the statement that Snape "glimpsed [Remus] at the end of the tunnel", and not at all with the statment that he would have encountered the werewolf if he had got as far as the Shack. Not getting as far as the Shack surely doesn't mean "was two feet from the entrance".
We can make sense of the idea of Severus glimpsing Remus both at the end of the tunnel and in the Shack if we get rid of the twist, but the twist is unambiguously described only one chapter before the description of the werewolf incident, and it still leaves the question of why James would be in any danger, since we know Severus did not get as far as the Shack. I suppose it's possible James feared Severus would loose Remus into the tunnel: but why would he?
Alternatively, if we assume that Sirius had loosed Remus into the tunnel then that makes sense of Severus needing to be saved, and (sort-of, of which more anon) of James being also in grave danger, and of Severus glimpsing Remus at the end of the tunnel – meaning that either Remus was loose at the top of the slope as Severus looked up it, or he had progressed to the bottom of the slope and Severus saw him from farther along the nearly-horizontal part of the tunnel – but it creates a problem with the statement that Severus would have met the werewolf if he had reached the Shack. But perhaps that can be reconciled if Remus was just at the top of the slope, only a few feet from the opening into the Shack, when Severus saw him. Severus glimpsed him from a distance but didn't directly encounter him, but if he had got much closer to the Shack he would have done.
Another issue, raised by Lydia A Campo on Quora, is how James and Severus got out. Unless he transformed into a stag, what could James do against were-Remus that Severus could not do? Remus says that Severus didn't get as far as the Shack, and I don't see why they would think Severus would actually go into the Shack, unless Sirius told him something that would make him do so, and absolutely definitely no-question meant Severus to be attacked by the werewolf and killed or infected. So we're probably not talking about James catching Severus near the Shack and preventing him from entering (and if we are we have the same problem about how Severus could see Remus in the Shack, without having reached the Shack).
Adult Snape doesn't seem to know that James was an Animagus: if he had known he would surely have said to Dumbledore "If Potter was an Animagus maybe Black was too and that's how he's getting into the school". So unless James Obliviated him, James did not transform in order to control Remus. Two human boys were trapped in a very long, low tunnel with a werewolf who could run on four feet, yet they got out. That suggests that in fact Severus and James were only a short way into the tunnel when they saw were-Remus and were able to chuck obstacles in his way to delay him while they climbed out by the Willow. But that, in turn, would definitely mean Remus had been turned loose.
My own theory – and it is just a personal theory, but it does fit what we're told – is that the boys collectively decided that they would allow Severus to glimpse the werewolf, in the hope that once his curiosity was satisfied he wouldn't keep looking and wouldn't find out that they were Animagi, and were letting Remus out; but Sirius then unilaterally decided to turn Remus loose, and James found out about it. We don't know whether James saved Severus just to keep Sirius and Remus out of trouble, or whether he genuinely drew the line at murder: but he certainly didn't do it for Sev's own sake, since by the end of the academic year he was back to persecuting him, and encouraging Sirius to do so.
Another possibility is that Madame Pomfrey did not accompany Remus all the way to the Shack but just to the Willow. Remus was meant to lock himself in in some way he couldn't open while in wolf form, but he left the entrance open so he could get out again to join his friends, and Sirius sent Severus down there knowing Remus would be loose, but not having released him himself.
What on Earth did Sirius think he would achieve? It surely wasn't to protect Remus himself from discovery, since there was little or nothing Severus could do with the mere fact that Remus was a werewolf, which he knew was already known to and sanctioned by the staff. On the contrary, Sirius's actions put Remus at severe risk of being expelled or possibly even executed, to say nothing of the life-long trauma which he would suffer if he killed or infected a classmate. The most likely explanation is that yes, they were already Animagi, or about to be, and since being an unregistered Animagus is a crime he wished to conceal that fact. That's assuming Sirius meant Severus to die, as adult Snape definitely believes. Remus certainly considers his death was a likely outcome.
This whole sequence, combined with the fact that Greyback had to be taken down by whacking him on the head, and that in the later Shack scene in PoA adult Snape wanted to control soon-to-be-werewolf Remus by physically binding him, strongly indicates that werewolves are highly resistant to direct magic. Otherwise, why would Severus, with his wand, need to be rescued by James, with his wand, from were-Remus, with paws?
Also, why was James supposedly in danger? Sev's comment that "They sneak out at night" implies that James, Sirius and Peter are already gathering information for the Map, which in turn suggests they are already Animagi, if the sequence of events which Remus gives for their fifth year is accurate: but if that's so James should have been able to protect himself from Remus just by transforming, so the only danger he would really be in would be that of Severus learning that he was an unregistered Animagus. Or maybe he actually can't transform in the tunnel, in which case "They would then slip down the tunnel and join me" would have to mean "Sirius and Peter slipped down the tunnel to get me while James waited at the edge of the Forest, near the Willow".
Whether James would fit down the tunnel at all is a moot point. If he really is a stag, not a buck, he's the size of a small horse. If he also has a big rack he may not fit into the tunnel at all, and if he does he will have to proceed either with his nose tucked between his front knees to bring his antlers down, or with his face pressed to the ceiling to bring his antlers back along his spine. The fact that Harry initially mistook the Prongs Patronus for a unicorn, and Aberforth managed to persuade some Death Eaters that it was a goat, suggests that Prongs in fact has little teenage-boy-deer antlers, which would be less of a problem: but he would still fit into the tunnel like a bung in a bottle, and would find it extremely difficult to turn round. So even if he was already an Animagus, he might have been in real danger because he needed to resume human form to perform some actions, including squeezing past Severus to fend off the werewolf if necessary.
It is very unlikely that James did in fact transform in front of Severus, unless he Obliviated him afterwards. Yes, he might have extracted a promise of silence in return for saving Sev's life, but if adult Snape had known that James was an Animagus he would surely have wondered whether Sirius was one too, and warned him to ward the school against Animagi in Harry's third year.
'Peter Pettigrew's dead!' said Harry. 'He killed him twelve years ago!' [cut] 'I meant to,' he growled, his yellow teeth bared, 'but little Peter got the better of me ... not this time, though!' And Crookshanks was thrown to the floor as Black lunged at Scabbers; Ron yelled with pain as Black's weight fell on his broken leg. 'Sirius, NO!' Lupin yelled, launching himself forwards and dragging Black away from Ron again, 'WAIT! You can't do it just like that – they need to understand – we've got to explain –' 'We can explain afterwards!' snarled Black, trying to throw Lupin off, one hand still clawing the air as it tried to reach Scabbers, who was squealing like a piglet, scratching Ron's face and neck as he tried to escape. 'They've – got – a – right – to – know – everything!' Lupin panted, [cut] Black stopped struggling, though his hollowed eyes were still fixed on Scabbers, [cut] 'All right, then,' Black said, without taking his eyes off the rat. 'Tell them whatever you like. But make it quick, Remus. I want to commit the murder I was imprisoned for ...' [PoA ch. #18; p. 256/257]
'Hurry up, Remus,' snarled Black, who was still watching Scabbers with a horrible sort of hunger in his face. 'I'm getting there, Sirius, I'm getting there ... ' [cut] 'That was still really dangerous! Running around in the dark with a werewolf! What if you'd given the others the slip, and bitten somebody?' 'A thought that still haunts me,' said Lupin heavily. 'And there were near misses, many of them. We laughed about them afterwards. We were young, thoughtless – carried away with our own cleverness. [PoA ch. #18; p. 259/260]
'Professor Snape was at school with us. He fought very hard against my appointment to the Defence Against the Dark Arts job. He has been telling Dumbledore all year that I am not to be trusted. He has his reasons ... you see, Sirius here played a trick on him which nearly killed him, a trick which involved me –' Black made a derisive noise. 'It served him right,' he sneered. 'Sneaking around, trying to find out what we were up to ... hoping he could get us expelled ...' [PoA ch. #18; p. 261]
'The joke's on you again, Severus,' Black snarled. 'As long as this boy brings his rat up to the castle –' he jerked his head at Ron, '-- I'll come quietly ...' [PoA ch. #19; p. 264]
'You shouldn't have done that,' said Black, looking at Harry. 'You should have left him to me ...' [PoA ch. #19; p. 265]
Harry went right after Black, who was still making Snape drift along ahead of them; he kept bumping his lolling head on the low ceiling. Harry had the impression Sirius was making no effort to prevent this. [PoA ch. #20; p. 277]
Sirius turned right around to look at him; Snape's head was scraping the ceiling but Sirius didn't seem to care. [PoA ch. #20; p. 278]
'Yeah, I camped out at your dad's in the school holidays, and when I was seventeen I got a place of my own. My Uncle Alphard had left me a decent bit of gold – he's been wiped off here, too, that's probably why – anyway, after that I looked after myself.' [OotP ch. #06; 103/104]
The only sources of light were the fire and one guttering candle, which illuminated the remains of a solitary supper. [cut] Sirius was hurrying towards them all, looking anxious. He was unshaven and still in his day clothes; there was also a slightly Mundungus-like whiff of stale drink about him. [OotP ch. #22; 420]
Lupin said quietly, 'I wouldn't like you to judge your father on what you saw there, Harry, He was only fifteen --' [cut] 'Snape was just this little oddball' [cut] 'Yeah,' said Harry, 'but he just attacked Snape for no good reason, just because -- well, just because you said you were bored,' he finished, with a slightly apologetic note in his voice. 'I'm not proud of it,' said Sirius quickly. [OotP ch. #29; 590]
'Of course he was a bit of an idiot!' said Sirius bracingly, 'we were all idiots! Well – not Moony so much,' he said fairly, looking at Lupin. But Lupin shook his head. 'Did I ever tell you to lay off Snape?' he said. 'Did I ever have the guts to tell you I thought you were out of order?' 'Yeah, well,' said Sirius, 'you made us feel ashamed of ourselves sometimes ... that was something ...' [OotP ch. #29; 591]
'They're the Montgomery sisters and of course they don't look happy, didn't you hear what happened to their little brother?' said Hermione. 'I'm losing track of what's happening to everyone's relatives, to be honest,' said Ron. 'Well, their brother was attacked by a werewolf. The rumour is that their mother refused to help the Death Eaters. Anyway, the boy was only five and he died in St Mungo's, they couldn't save him. 'He died?' repeated Harry, shocked. 'But surely werewolves don't kill, they just turn you into one of them?' 'They sometimes kill,' said Ron, who looked unusually grave now. 'I've heard of it happening when the werewolf gets carried away.' [HBP ch. #22; p. 442]
Did Sirius truly intend Severus to die? Adult Snape certainly thinks Sirius meant to kill him, and Remus considers that both Severus and James were in grave danger. On the other hand we are told that it's actually unusual for a werewolf to kill rather than just infect their victim. On what a friend of mine calls "the third or Sellafield hand", if Sirius didn't mean Severus to die, what the hell did he think would be gained by turning Severus into a werewolf too, resulting in his being there whenever Remus transformed?
[Sellafield, for those who don't know, is a large British nuclear-waste-processing plant and former nuclear power station which, under its earlier name of Windscale, was the site of a major nuclear accident in 1957.]
The worst interpretation of Sirius's actions, which would be sane but unpleasant and which I am not strongly espousing, is that he was angry with Remus for having protested at the way he and James were treating Severus, and so decided to punish him by weaponising him and making him the instrument of Severus's death. People like to think of the Marauders as this band of brothers, four boon companions roaming the woods together in beast form, but Sirius and Remus would grow up to each believe the other was the traitor in the Order, and Peter betrayed James to death and Sirius to Azkaban. Really, they were not four good friends, but James and three people who were each separately friends with James, but not much with each other.
Nor does the fact that Remus forgave Sirius for the werewolf incident prove anything, since Remus wanted to be let out to run, and to do so safely he had to remain subordinate to the others, and probably specifically to Sirius. Wormtail was too small to steer him and Prongs was a prey animal, but Padfoot was a Newfoundland (the only large, bear-like dog which commonly comes in black with yellow eyes, and is capable of swimming the North Sea), so Remus relied on Sirius to be able to control him in his wolf form.
It is definitely the case that Sirius is a would-be killer, whether he intended Remus to kill Severus or not. He intended to kill Peter when he realised Peter had betrayed the Potters and, OK, that was during a war. But when he escapes from Azkaban he slashes the Fat Lady: and the portraits are not just canvas and paint but are sapient persons able to feel pain and fear. We don't know if he slashed the Fat Lady's person or just her background but he certainly terrorised her, and this was not really different from threatening, say, Madame Pince with a knife. In the Shack he is still saying that Snape deserved whatever he had tried to do to him 18 years earlier, and he declares that he is there to commit the murder for which he was imprisoned. He is obviously serious about wanting to kill Peter in front of three children, and yearns for the killing "with a horrible sort of hunger".
OK, Sirius has a deep grievance against Peter and knows Peter is guilty, but he ought to realise that Snape sincerely believes that he is guilty. He must realise that Snape thinks he betrayed James and Lily and killed Peter and twelve Muggles, but he is offended by Snape's anger towards him and brutalises him while unconscious, bumping his head against the ceiling and risking triggering Second Impact Syndrome, which is instantly fatal. And OK, it's dark and they're in a low, narrow space and Sirius is long out of practise and using somebody else's wand, but Harry certainly has the impression that he at best doesn't care about hurting Snape. When his very life depends on getting Snape to calm down and listen to him, he can't resist jeering at him and reminding him of how he nearly killed him ("The joke's on you again"), and who knows what he meant by 'You should have left him to me'.
However, in PoA he is disturbed by nearly twelve years in Azkaban and nearly thirteen spent mostly as a dog, and he was never Mr Stability to begin with: he is after all Walbuga's son and Bellatrix's cousin, and will go on to die of being a grandstanding idiot. It's quite likely that when he tried to weaponise his boyhood friend to murder or infect his bullying victim he was having some kind of mental breakdown, so we cannot assume that he really had a sensible reason, as opposed to a disturbed whim.
It's important to consider the timing. We know that the werewolf incident happened in fifth year, and no earlier than the middle of the first term, because it was when Sirius was sixteen, so after his sixteenth birthday on 2nd November 1975, and it was before the scene by the lake which happened at the end of fifth year in June 1976.
As explained here, we know that James and Sirius bullied Severus "relentlessly" and that in the scene by the lake, bullying Severus is already a very well-worn groove: something James is sure will liven Sirius up, and Peter expects will give him pleasure (his expression is “avid”). We also know that by seventh year James had stopped bullying Severus four on one (although they still sparred one on one) and was an Order member, suggesting that he grew up during sixth year and that the lake scene was probably towards the end of his bullying period, not the start.
So, in the lake scene Severus has already been subjected to many attacks by the Marauders yet he is not on the lookout for them – despite having just answered a question on werewolves – and sits down near them without concern. The implication is that they have left him alone for long enough that he has begin to feel safe. Why would they have left him alone, when tormenting him is implied to be Sirius’s favourite sport?
At this point I’m making an assumption, but I think a reasonable one. I assume that the most likely reason for them to have left Severus alone for long enough for him to start to feel safe is that they stopped hounding him after the werewolf incident – either because Sirius spent months in detention, if the staff knew about his part in it, or because knowing that they had nearly killed him genuinely sobered them up for a bit. That sets the werewolf incident some months back before SWM, long enough for their victim to begin to feel safe. The fact that Remus says that James was fifteen during the SWM scene by the lake, when really he was sixteen (his birthday is in March), may mean that Remus's mind tends to jump back to the werewolf incident and that James was fifteen then, which would place the incident earlier than 27th March.
It also means that the well-worn groove was worn before the werewolf incident. Sirius came to it already habituated to seeing Severus as something less than human, a mere toy for him to amuse himself by kicking around, and angry that his toy was trying to fight back by getting something on them. And it happened at least some months before June.
It also makes sense that it would have happened during winter, because given that they have a curfew, and the Slytherin dorm overlooks the lake, not the Willow, it’s far more likely Severus would have seen Poppy taking Remus to the Willow if it was happening at 3pm rather than 9pm.
Then, we have to make two more assumptions, less definite but still reasonable, in relation to Sirius’s statement that "I camped out at your dad's in the school holidays, and when I was seventeen I got a place of my own." The first is that when Sirius says he bought a flat when he was seventeen he means as soon as possible after he turned seventeen, meaning he was in his own place by Christmas 1976.
The second is that when Sirius says that he split from his family in fifth year and then spent "the school holidays" with the Potters, he is referring to more than one holiday period. This is only very weakly implied because Britons often refer to any holiday period more than a day or two long as "the holidays", plural, and Rowling habitually does so. All I can offer is that if Sirius – and Rowling – meant us to understand that this happened only once then Sirius would probably say "the summer holidays".
So, if we allow those two assumptions, Sirius must have spent at least the summer holiday between fifth and sixth year and the Easter holiday of fifth year with the Potters, and maybe the Christmas holiday as well. That means his split from his family happened prior to Easter 1976 (very possibly at Christmas, since that’s such a fertile source of family friction). That puts it in the same time-frame as already established for the werewolf incident.
Whichever of them happened first (and it’s possible the final split with his family was because he had nearly killed a classmate, or because he was friends with a werewolf), the werewolf incident happened at a time when Sirius was seriously at odds with his family, under a great deal of stress and possibly having some kind of mental breakdown. He was annoyed that Severus was watching them, and thought that Severus was trying to get them expelled – and resented it, despite the fact that Severus had every reason to want to get them off his back and out of his life. How dare their toy fight back? So he probably acted out of stupid spite, without really thinking the consequences through.
A few weeks before his death Sirius says that he isn't proud of how he and James treated Snape, but the circumstances are such that it's likely he is only saying that to mollify Harry, and even in this conversation he justifies his persecution of a "little oddball". There's no indication that he ever truly feels any remorse for his bullying, or for the mureder attempt (if that's what it was).
However, that's not really surpising. He was arrested the day before his twenty-second birthday and spent nearly twelve years in Azkaban, falsely accused of betraying his friend. He protected himself from the Dementors for those twelve years with the knowledge that he wasn’t guilty of the crime for which had been gaoled – but that meant he probably couldn’t afford to admit to his own guilt in the crimes he had committed. He had to keep telling himself he was innocent and justified.
The he escaped, but he only lived another three years, most of which he spent as either a dog or stuck in a house he hated, cultivating an implied drink problem and being screamed at by the portrait of his mad mother. He never really got a chance to grow up. Also he had lost James, and it was partly his fault because it had been his idea to make Peter the Secret Keeper. Persecuting Snape was how he and James first bonded, and if he admits that he was wrong to torment Snape, he has to admit that James was wrong too.
'Quirrell said Snape –' 'Professor Snape, Harry.' 'Yes, him – Quirrell said he hates me because he hated my father. Is that true?' 'Well, they did rather detest each other. Not unlike yourself and Mr Malfoy. And then, your father did something Snape could never forgive.' 'What?' 'He saved his life.' 'What?' 'Yes ...' said Dumbledore dreamily. 'Funny, the way people's minds work, isn't it? Professor Snape couldn't bear being in your father's debt ... I do believe he worked so hard to protect you this year because he felt that would make him and your father quits. Then he could go back to hating your father's memory in peace ...' [PS ch. #19; p. 217]
Snape set down the smoking goblet, his eyes wandering between Harry and Lupin. 'I was just showing Harry my Grindylow,' said Lupin pleasantly, pointing at the tank. 'Fascinating,' said Snape, without looking at it. 'You should drink that directly, Lupin.' 'Yes, yes, I will,' said Lupin. 'I made an entire cauldronful,' Snape continued. 'If you need more.' 'I should probably take some again tomorrow. Thanks very much, Severus.' 'Not at all,' said Snape, but there was a look in his eye Harry didn't like. He backed out of the room, unsmiling and watchful. [PoA ch. #08; p. 117/118]
'I sometimes felt guilty about betraying Dumbledore's trust, of course ... he had admitted me to Hogwarts when no other headmaster would have done so, and he had no idea I was breaking the rules he had set down for my own and others' safety. He never knew I had led three fellow students into becoming Animagi illegally. But I always managed to forget my guilty feelings every time we sat down to plan our next month's adventure. And I haven't changed ...' Lupin's face had hardened, and there was self-disgust in his voice. 'All this year, I have been battling with myself, wondering whether I should tell Dumbledore that Sirius was an Animagus. But I didn't do it. Why? Because I was too cowardly. It would have meant admitting that I'd betrayed his trust while I was at school, admitting that I'd led others along with me ... and Dumbledore's trust has meant everything to me.' [PoA ch. #18; p. 260/261]
Snape glimpsed me, though, at the end of the tunnel. He was forbidden to tell anybody by Dumbledore, but from that time on he knew what I was ...' [PoA ch. #18; p. 261]
'You know how and why she died. Make sure it was not in vain. Help me protect Lily's son.' 'He does not need protection. The Dark Lord has gone –' '-- the Dark Lord will return, and Harry Potter will be in terrible danger when he does.' There was a long pause, and slowly Snape regained control of himself, mastered his own breathing. At last he said, 'Very well. Very well. But never – never tell, Dumbledore! This must be between us! Swear it! I cannot bear ... especially Potter's son ... I want your word!' 'My word, Severus, that I shall never reveal the best of you?' Dumbledore sighed, looking down into Snape's ferocious, anguished face. 'If you insist ...' [DH ch. #33; p. 544/545]
'You have guessed, I know, why the Cloak was in my possession on the night your parents died. James had showed it to me just a few days previously. It explained much of his undetected wrongdoing at school!' [DH ch. #35; p. 572]
It seems only right to make one small, additional comment on Professor Dumbledore's notes. As far as we can tell, the notes were completed around eighteen months before the tragic events that took place at the top of Hogwarts' Astronomy Tower. Those familiar with the history of the most recent wizarding war (everyone who has read all seven volumes on the life of Harry Potter, for instance) will be aware that Professor Dumbledore reveals a little less than he knows – or suspects – about the final story in this book. The reason for any ommission lies, perhaps, in what Dumbledore said about truth, many years ago, to his favourite and most famous pupil:
What did the staff know about it? We know Dumbledore knew that Severus had been down the tunnel and probably that he had actually seen were-Remus, because he swore him to secrecy about it. In one way this was reasonable, because as far as we know Remus was an entirely innocent party in this instance; but it meant that Severus, already a survivor of extreme physical abuse according to Pottermore, had had a traumatic, life-threatening experience and then wasn't allowed to talk about it, even to his friends. It's not surprising that adult Snape is very jumpy and shows signs of PTSD, and is still so scared of Remus that when he brings him his Wolsfbane he backs out of the room, as if he daren't turn his back on him.
We know that by the end of Harry's third year Dumbledore does know that Sirius sent Severus down under the Willow. But perhaps his rather cold reply to a reminder from Snape is because he had believed Sirius guilty of betraying the Potters and killing Peter and twelve Muggles in part because Snape had told him about his own attempted murder, and now Dumbledore knows Sirius is probably innocent of betraying the Potters, and resents the fact that Snape had encouraged him to accept Sirius's guilt.
I'm not saying the staff necessarily didn't know at the time that Sirius had set Severus up: just that the matter is left open. The fact that Sirius wasn't expelled isn't strong evidence that they didn't know, because if he had already split from his family there was nowhere to expel him to, and he was still a minor so they couldn't just dump him on the street. There seems to be nothing like juvenile detention in the wizarding world. The fact that Severus is not on the lookout for the Marauders after his DADA OWL suggests they have left him alone for a while, which may mean that the staff did know and that Sirius had spent months in detention every night.
We do know that Dumbledore was not aware at the time that the Marauders were Animagi, and apparently not about the Map. He also didn't know James had Death's own perfect Invisibility Cloak, and he says that when he found out about it it "explained much of [James's] undetected wrongdoing at school". That suggests that there were things James, and probably Sirius, were suspected of but that couldn't be proven at the time. Dumbledore certainly wouldn't know for sure that Sirius had set Severus up unless one of them told him so: and despite what Sirius thinks about Severus trying to get him expelled, Severus might well not have told on him at the time, until Sirius was arrested for mass murder six years later, because telling tales to the teacher Isn't Done.
It may also be that the Marauders used the Cloak and the Map to usually get Severus where there were no witnesses, meaning that even if he tried to tell somebody what was happening he wouldn't be believed, because having Death's mythical Cloak would be like a kid at a real, Muggle school having Caliburn (Excalibur). Dumbledore might be genuine when he speaks as if it is odd of adult Snape to resent James for saving his life, and may really not know (yet, although he does know two years later) that it was James's gang who put him in danger in the first place. And he has promised Snape not to tell Harry why Snape is really protecting him, so he may just be paltering, saying something which is true as far as he knows, but a bit misleading.
Or he may just be lying. Dumbledore is what we call here "economical with the truth", and Rowling openly gloated about it in The Tales of Beedle the Bard. After showing Dumbledore writing an essay in which he claims he is not one of those foolish people who think the Hallows might be real, written when he had the Wand, knew where the Cloak was and was hot on the track of the Stone, she reminded readers of what Dumbledore said about truth in the first book – effectively saying "Haha, I told you he was a liar in the first book, and you didn't notice!". Except that some of us did.
