A/N: I actually can't remember if it's book canon or not, but I just rewatched PoA, and there's a scene where Harry closes the compartment door on the Hogwarts Express so that he can tell Ron and Hermione that Sirius Black is hunting him.
At this time, of course, they're sharing the compartment with a sleeping "Professor R. J. Lupin," but I realized as I watched it that Peter Pettigrew was also in that same compartment! So this idea began in my head of "two Marauders listening to a third Marauder's son talk about the fourth Marauder", and I just had to attempt to capture it.
It's short and sweet. Let me know if you like it!
Remus Lupin wasn't asleep. In fact, he was wide awake when the son of his dead best friend traipsed into his compartment on the Hogwarts Express. He was ashamed to admit that he froze with the realization of who this dark haired, green eyed young man was. Remus was unable to move, or talk, or otherwise do anything to indicate to the teenagers that he wasn't asleep. And so, when Harry shut the compartment door to debrief his classmates on a dangerous secret, Remus laid in a faux slumber, unintentionally eavesdropping.
He listened in shocked silence as Harry told his terrified friends of Sirius Black, a name that drove a painful pang through Remus' heart. He listened to Harry's confusion over Sirius' escape from Azkaban, and from what the poor boy had been told, his old friend's insane desire to murder the child.
He listened with tears streaming down his face, and was thankful that his cloak covered it, because he knew that he could never explain. He would never have the courage to look Harry in the face- that face which looked so like his father's, but with Lily's expressive and loving eyes- and tell him that he knew, in heart breaking, painstaking detail, why Sirius Black was actually after him.
And Lupin's heart, already battered and beaten, broke a little bit more.
Peter Pettigrew wasn't asleep. In fact, he was wide awake when the son of his dead best friend stood to shut the compartment door on the Hogwarts Express. He was not ashamed to keep his form of a rat, sitting perfectly still on the Weasley boy's shoulder and pretending that he was nothing more than a good and loyal pet, as he listened to Harry debrief his classmates on a dangerous secret.
He listened in nervous silence as Harry told his terrified friends of Sirius Black, a name that drove a deep-rooted pang of fear through Peter's heart. He listened to Harry's confusion over Sirius' escape from Azkaban, and mentally congratulated himself that his decade old story was still perceived as fact, and that the boy had been warned only of Sirius' insane desire to murder.
He listened with his heart beating rapidly in his tiny ribcage, and the irony that Remus Lupin, of all the people in the entire world, sat only feet away from him was not lost. Peter knew that he could never explain it to Remus; that he'd never be able to tell his old friend how it'd really happened, all those years ago.
And Harry- who looked so like his father, but with Lily's innocent and trusting eyes... Peter was ashamed to admit that when he felt regret, it was for all the wrong reasons. He only regretted that when he'd finally made what he'd considered to be a calculated move in order to preserve his own life, it'd backfired. He only regretted that it seemed, in hindsight, that he'd chosen wrong. In trying to take the easy path, he'd made his new path more difficult to follow.
Deep down, Peter also knew he would have never had the strength to choose differently back then, even given the chance to try. But sometimes, when he was alone, trapped forever in this form of a tiny rodent and unable to run away from even his own consciousness, he wished that he'd been braver.
And so Peter did what he always did in these scenarios. His heart, already callous and unfeeling, hardened a little bit more.