The interaction between the Marauders is absolutely fantastic. I love that you've included Peter in the scene, and that he seems to be fully a part of the conversation, and not just a tacked on addition.
Fantastic maneuvering between Sirius and Lucius; love all of the implications there. Your Lucius is smooth and perfect. The entire story being told through the filter of James is brilliant.
If there was one moment that James shouldn’t have watched, if he insisted on watching at all, it was this one, this one moment, barely a few seconds in length, when Lucius gripped Sirius's hip with one hand and spread him with the other, pushing inside in one smooth stroke. It was a moment that, once watched, could not be unwatched, and once seen could never be unremembered.