Author's Note: Written at the suggestion of a guest reviewer. (I don't actually know Prisoner of Azkaban very well, as I managed to somehow skip right over it when I was reading the series for the first time and never quite picked up the taste afterwards.)
I will say that that POA hangs together better than the other books because it relies on several people making bizarre decisions while emotionally devastated or caught in a situation when events are moving faster than their brains. That said, this scene is... weird.
Content warning for offscreen violence.
"What about Professor Snape?" said Hermione in a small voice, looking down at Snape's prone figure.
"There's nothing seriously wrong with him," said Lupin, bending over Snape and checking his pulse. "You were just a little — overenthusiastic. Still out cold. Er — perhaps it will be best if we don't revive him until we're safely back in the castle. We can take him like this…"
He muttered, "Mobilicorpus." 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 pup pet. He hung a few inches above the ground, his limp feet dangling. Lupin picked up the Invisibility Cloak and tucked it safely into his pocket.
"And two of us should be chained to this," said Black, nudging Pettigrew with his toe. "Just to make sure."
"Chained?" Lupin looked oddly at Black. "Sirius - you know very well chains can't hold an Animagus. Especially not one with his form."
Black grimaced. "Suppose I've spent too much time thinking like a dog." An unwholesome expression stole across his features. "Say - he didn't regrow that finger he lost, did he?"
Harry grew alarmed at the way he eyed the prone Pettigrew. "I don't think my dad would have wanted that, either-"
"Now, now, Harry, that's not what I meant," Black said, the expression receding from his features. Harry didn't like the trace that remained. "My mother would, but not me. No, we won't do any permanent damage..."
One conjured cricket bat later, Pettigrew entered the tunnel in much the same condition as Snape, save that his legs hung limply and... bent a few different ways. Hermione looked pale.
Ron, dragging his own broken leg, looked unsympathetic. "I wouldn't mind being knocked out," he muttered. "I think he's getting off easy."
"We have to be better than our enemies, Ronald," she hissed under her breath, "and I don't think all of that was really a necessary precaut-"
"Necessary precautions..." Lupin breathed, then went white as a sheet. Without explanation, he turned tail and started sprinting back down the tunnel. "Sorry! Very sorry! Go on without me, everyone!"
"Forgot my potion! I'm so sorry!"
There was dead silence as Lupin disappeared into the Shack. "We've got to go," Black said grimly. Harry looked about at their sorry lot: three students, one injured, one unconscious professor, one unconscious, viciously-battered man who was supposed to be dead, and one prison escapee... Something brushed by his feet, and he jumped. Right - and one too-smart-for-its-own-good cat.
Could anyone accept their story? It was too fabulous to be believed, he could scarcely believe it himself... It was hard to fathom that -
"I said move," Black snarled. "Unless you'd like a close encounter of the wolfy kind?"
It was remarkable how fast one could move when properly motivated. Even Ron managed to set a good pace with his arms around Harry and Hermione's shoulders, hopping along in a perverse kind of three-legged race; they, and the unconscious men, cleared the tunnel just in time, for a sort of horrible howling and screaming started up the moment they were up and out in the open air. Black grimaced, then turned his gaze to the silhouette of Hogwarts Castle.
"There's nothing we can do for him," he said, looking grim. "The only thing we can do now is get to the castle, and put an end to this sorry mess once and for all."
Guest: Maybe your next chapter could be about how Pettigrew's escape at the end of Harry's third year could have been prevented if Lupin and Sirius had just stunned him and levitated his unconscious body the way they did with Snape Instead of chaining him up but leaving him conscious.
They could even have cast a full body bind on him so that even if he did wake up, he would've been unable to move.
I favored the 'broken bones' approach because Animagi are explicitly shown not to be able to immediately heal from injuries, but the Guest's approach also works assuming there's no way to break out of a Full-Body Bind. Thanks for the suggestion!
Alternatively, since we know from HBP that Harry has the amazing ability of super-fast speech (he gets out "Sectumsempra!" before Draco can manage "Crucio!"), here's a slightly-later divergence in which Harry's faster on the draw. (I think he could get out the required incantation in the time he says "Expelliarmus! Stay where you are!" anyway...)
Harry stood, transfixed by the sight, too intent upon the battle to notice anything else. It was Hermione's scream that alerted him — Pettigrew had dived for Lupin's dropped wand. Ron, unsteady on his bandaged leg, fell. There was a bang, a burst of light — and Ron lay motionless on the ground. Another bang — Crookshanks flew into the air and back to the earth in a heap.
"Petrificus Totalis!" Harry yelled, pointing his own wand at Pettigrew; the man went rigid, and fell over on the spot. Wishing desperately that he knew some more powerful spell to use upon him, Harry ran forward and bent down to wrench Lupin's wand from Pettigrew's hand. He half-fancied that Pettigrew's body was shrinking, and performed upon the man a dirty fighting technique which Dudley had gleefully performed upon him on multiple occasions: it amounted to kicking someone when he was down... strategically. It seemed to work - if Pettigrew had been attempting to transform, he wasn't any more. Harry would consider it a wonder if the man could manage coherent thought, really. He had not been gentle.
Harry took a quick look about: Hermione was kneeling by Ron, the werewolf had made its escape, and Black was huddled on the ground, muzzle and back bleeding. Some whining, childish part of him remarked that of course he was ending yet another year with the adults having left him in the lurch at the critical moment...
Well, it wasn't as though he hadn't lived through worse before, he thought grimly. And that just summed up his life, didn't it?