Sirius starts wearing eyeliner in the summer before their 5th year, having found a pen lying around in a muggle park. So the first day back to school, him and James stroll back into the common room, and Remus is already there, of course, having arrived in the Hogwarts Express while Sirius and James (much like Harry and Ron) had to arrive by other… means.
And Sirius is used to the eyeliner by now, having been using it all summer, so when Remus goes to greet him and James, and he just stares, Sirius says, “what?”
Then Remus is just, like, staring, his eyes taking in every detail on Sirius’s face, always going back to his eyes.
And now Sirius just stands there, awkwardly, and says like “Moony what the fuck?”
So then James says, “Pads… your eyes,” trying to stiffle a laugh, and sirius just beams when he realizes, and says, “OH RIGHT! DON’T YOU LOVE IT MOONY? EYELINER! ON A GUY! ISN’T IT AMAZING? THE POTTERS WENT CRAZY WHEN THEY SAW IT!” all excitedly and practically shoving his face against Remus’s.
So Remus now is looking like he might faint, and Sirius’s excitement dies down a little as he says, “Oh… you don’t like it?” and Remus is still saying nothing, so James groans and gets between the two of them and slaps Remus softly. “Remus. Stay with me. Sirius asked you a question, honey,” and Remus manages to strip his eyes from Sirius’s face to James’s, then back go Sirius’s, and he just moves past james to where Sirius is standing, grabs his face, leans in really close, and says, in a breathy, rough whisper, “You’re fucking killing me, Sirius.”