Sirius first noticed something was wrong when Remus didn't return his kiss. That was unusual and prompted him to check Remus' expression. That was when he saw the mask of horror fixed over Remus's face. Wide eyes stared over Sirius's shoulder. Those scrumptious lips moved soundlessly. Remus looked as if he had just seen- well, not a ghost, but something worse.
Sirius turned slowly, his heart hammering. Of everything in the world he could have imagined standing behind him- Fenrir Greyback or a Hungarian Horntail- he had not imagined this.
"James," Sirius answered flatly.
"You two were snogging!" James's tone was accusatory.
"Yeah, did you want to join, or-?"
"Merlin, no!"
"Then please let me and Moony get back to it. I'll be up for a kitchen raid after, I promise."
James still looked shocked, but he had the good sense to shove his hands in his pockets and continue walking down the corridor.
"That went better than I expected," whispered Remus.
"James is a mate," Sirius answered, before pining Remus's face between his fingers and continuing the snogging right where he'd left off.