Disclaimer- I know you're all under the impression that J.K Rowling writes fan fiction for her own books on this site and that this is one of the many she has posted, but that is sadly not the case (though I wish it was) ^_^

Peter Pettigrew idly sat on a musty sofa in a room above the three broomsticks. His pale blonde hair hung about his round face, and he hugged himself with short, chubby arms as though for protection. His eyes flitted about the small room, as though he suspected that somebody would jump out at him any second. Discarded butterbear bottles were strewn over the floor, and plates of unfinished food had been left on every available surface. His forearm still burned where The Dark Lord had put his mark, though the actual ordeal had taken place several weeks ago. He shivered to think about it, and hugged himself tighter, as though that would protect him.
Shakily, he pushed himself up off the couch and walked towards the fireplace, where he conjured up some lively flames with his wand. He stared at them, eyes reflecting the bouncing tendrils of light. And then, on impulse, he slowly pulled up the sleeve of his robe. There it was, the cold skeletal face with a serpent in place of a tongue. He stared at it with sickened fascination, as though the arm was not his own but somebody else's. The light of the fire dance around the simple black lines that bonded him with his master.
"Alright there Wormtail?" A mocking voice said from behind him.
Peter squeaked and spun around, yanking down the sleeve of his robe. "S- Sirius!" he exclaimed.
"No, no... Voldemort's not here to get you. You're not important enough for him..." Sirius remarked. But despite the light words, Peter noticed the dark circles under the other man's eyes, and his unusually pale face.
Peter shuddered to hear the name of his master spoken aloud, and winced, though Sirius did not notice. In fact, Sirius never seemed to take notice of him at all, other than the occasional jibe directed at him. "So er... sit down," Peter said, gesturing at the couch he had been sitting on.
Sirius did so, though there was an unusual wariness in the way he sat down.
Peter expected his friend to make some comment about the disarray of the place, but he didn't. His mind was preoccupied with other, darker things.
"Well, Peter," Sirius began.
Immediately a trace of fear wound it's way through Peter's mind. The marauders never used each other's real names unless something was wrong. Had Sirius found out about The Dark Lord and his new servant? The mere thought about it was enough to make his hands shake, and the dark mark upon his forearm tingled. "Yes?" Peter asked.
"I, well... it's about James, and well, you know... I think they're still in danger..."
"But," Said Peter as he uncorked a bottle of butterbeer for the sake of doing something, "They can't still be in danger from he-who must not-"
"Peter," Said Sirius darkly, "Why do you fear his name?"
"I..." But Peter did not, could not, finish.
"Well," Sirius continued, giving him a scrutinizing glance, "The thing is, well, everybody knows James and I have been... very close friends. And, I don't know... Voldemort obviously has somebody tracking the Potter's movements-"
Peter winced at this, but did not say anything.
"And I think that the spy would have noticed James and my friendship." Sirius was toying with a loose thread as he spoke, and he did not look Peter directly in the eye.
"So..." Peter said, "Your point is?"
Sirius took a deep breath, as though bracing himself for some near impossible obstacle.
"My point, Peter, is that I am not a good secret keeper. Gods knows I would do anything to protect him and his family, and if it weren't for the fact that our friendship is so well known..." Sirius paused here, and gripped his knees as though for support. "I've talked to James, Peter, and I, we, think that it would be better for you to be the secret keeper."
Immediately a torrent of raging emotions rushed through Peter, and combated against one another for dominance. This was it; if he was the Potter's secret keeper, he could tell The Dark Lord everything, he could hand over the Potter's lives and become The Dark Lord's most valuable servant. And then there was his love and admiration for James. He alone had been given the opportunity to protect them. He, Wormtail, always the "stupid" one could save, of all people, James, Lily, and Harry.
As if to remind him of his allegiances, the dark mark seared across his forearm, and grimacing, he absentmindedly clutched it.
"What is there to think about?" Sirius snapped; apparently he had noticed the grimace, and the long silence that had preceded it. "If I were you, which thankfully I most certainly am not, I would jump at the chance to help James in this way! Besides," he said quieting slightly, "you would be the last person Voldemort would even think to check up on."
A delicious anger immediately filled Wormtail, and all of a sudden, he was remembering every single nasty comment he had had to endure when in the company of Sirius and James. They had been the two most popular boys at Hogwarts in their time, and he, Wormtail, had been just another anonymous figure in the mass of students. Like everybody else, he had been drawn to James and Sirius, and he had tried to be like them. He had been ecstatic when they, and Remus of course, came up with the idea of the Marauders. He had felt important then. He was one of Sirius and James's inner-circle! But now, he saw looking back, they never really had thought of him as an equal. He was not good-looking, particularly witty, up to their intelligence...
This put his choice into new perspective. Be belittled his entire life by those whose lives he could save, or, to be the most powerful wizard's most trusted companion. "I'll do it!" He heard a voice besides his own say in answer to his question.
A tiny smile flickered over Sirius's face. "I knew you would, Peter," he said clapping the smaller man's shoulder, "Marauders 'til the end!"
"Yes," Peter said, noting the new meaning behind their old slogan, " 'til the end."