Mostly a dialogue, inspired by a comment that I read somewhere (with which I don't fully agree) about Harry's generation being mirrors of the marauders—and Neville being Peter. The claim is that Neville made all the (correct) hard choices, while Peter gave in to cowardice. There are, however, several key differences between them and if it was JKR's intention to make Neville the Improved Peter, then she did not write Peter very well. (Which, of course, is also possible.)
I have no idea when this is taking place. Perhaps Remus discovered Peter after taking the Marauder's Map and had the opportunity to confront him alone. Fuckin' drabbles, man.
If you squint and tilt your head, it's canon. Evidently nothing is mine but the story.
- — • — -
When he dispensed with the spitting and fidgeting, Remus could almost see a rundown shadow of the sharp young man Peter Pettigrew had once been.
"Something in me knew, Moony." Remus couldn't tell whether he was more disgusted or ashamed to hear the old nickname leave those lips. No one had called him Moony in over a decade. "This was going to happen. Some force must always have known. After all—I'm a rat." He looked tortured but Remus shook his head derisively, a quick, jerky movement. Dismissive.
"Don't blame this on something as trifling as physical form, Peter. Don't. I cannot listen to that."
And Peter shook, because how could Remus understand? He had known since he had first discovered his Animagus form that something, intrinsically, was wrong. It was a fact that the Animagus's animal form was determined by the wizard's character, by subtle traits that comprised their personality—and something inside Peter had spoken to The Powers That Be, had whispered behind his back... and they had whispered that he was a rat. It had consumed him since then. However the werewolf may scorn, consider it stupid, "trifling", it had worried at Peter for years. Eaten away at him, as it were. In all his reflections, he saw himself as lesser. Sirius's animal form, so obviously loyal. He had no idea what James's stag represented but it was certainly impressive. And lo, next to them all, the rat.
For years he had dwelled, had worried. Peter knew that he was a person anxious and nervous by nature and had wondered, at first, if perhaps that neuroticism was enough to inspire his less-than-desirable bestial manifestation. The thought had comforted him only briefly, until he had decided that there was more to it than that. Someone with a stronger character would not be so easily... ratted over something as small as a tendency to nervousness! No, it was deeper. Some mysterious rat-like quality as yet undiscovered even by himself.
Peter Pettigrew could, of course, forget that he was a rat. When he was with his friends, enjoying their company and even able to help them with his considerably smaller and more flexible form, he could appreciate being a rat. He had spent weeks nose-deep in books about rats, researching their positive qualities and deciding how applicable they were to his personality. And, generally, they were. But then there were subtleties to the rat trope itself that he could not ignore and magic was nothing if not subtle. "Rat" was neither a flattering moniker nor figure. That haunted him, and he would be lying if he said that he did not dwell on his potential darkness rather than his ingenuity or adaptability.
He wouldn't go so far as to say that he had consciously recognized his rat-like cowardice when confronted by Death Eaters for the first time. It wasn't as though he had looked his enemy in the face and thought, "This is the moment. This is what I've been dreading since I was 15. I'm going to betray everyone I love." It wasn't that distinct. But in hindsight the events following the initial threat were tinged with a sense of inevitability—a faint aftertaste of self-fulfilling prophecy. When Voldemort himself confronted Peter, and Pettigrew looked up at the man who had been Tom Riddle with tears running down his face, he had agreed to do anything if only his life could continue. He was baffled that, of all the Marauders, Voldemort had noticed him and all he wanted was to survive. It seemed impossible to say now, but he had been fighting to save his old life and his old happiness—he hadn't known that he could never have that again.
By the time he was asked to aid in the murder of the only people who might have protected him, his brave and strong and smart friends, Peter was so far down the rabbit hole—or rat hole, as it were—that he could no longer back out. And the rat has an extraordinary sense of self-preservation.
Of course Remus didn't want to hear that.
Peter didn't want to hear it.
He was looking into the face of one of his best friends, someone he had loved since his school years, and both of them knew that he had caused the deaths of two people whom they had treasured. Not just indirectly, either, but unequivocally. Peter understood that his life was forfeit in Remus's eyes and he didn't even think that Remus was wrong—but he had come this far already with the sole purpose of preserving that worthless heartbeat. He hadn't sold his soul to Voldemort only to allow himself to be killed at the will of the people he, grotesquely, still cared for. No. Peter would continue to fight against them as long as he lived... in order to live.
He sometimes shuddered to think that that would be his legacy, but there wasn't really time under the constant threat of death to consider what people would say about you when you were gone.
And Peter certainly couldn't justify himself to Remus. Not when there were lives lost and still on the line—not when the werewolf was standing across the room, looking at his old friend as though he did not know him. That was reasonable, of course, as Remus really didn't know Peter anymore. He was struggling to rationalize, to minimize the discrepancy between the happy young man he had known and the wasted traitor now before him. When Remus looked up again and his eyes locked with Peter's, there was no love there and Peter breathed deeply.
In a short moment, a rat stood where Peter had been—and another tick of the second hand later, the rat scrambled under the door and out into the Hogwarts grounds, leaving Remus frantic and enraged.