“Morning, Lupin,” Sirius greets a bit stiffly. He doesn’t quite know why he’s last-naming him. Maybe to try and force himself to put some distance between them, maybe to see if it unsettles Remus, if it gets his attention. It does, for a millisecond. Sirius receives a quick searching look that’s barely there and immediately gone.
“Good morning, Sirius,” Remus answers, and there’s a pathetic tingling in Sirius at hearing his first name. Like Remus chose not to last-name him back, like he’s going out of his way to refuse Sirius’ attempt to put distance between them. Like it’s some coded but meaningful gesture.