When Remus said surprise me, he wasn't looking for an actual surprise. He only meant for Sirius to give him some sort of hello that he didn't know beforehand and a kiss fell into that category easily enough. But leave it to Sirius to take things to a whole new level.
Honestly, he could go on kissing Sirius all day - or night. Didn't people make out all the time? He could see why, and the variation was pleasant and actually surprising by itself. However, he almost stopped completely when he realized what had happened. His arms gave a few experimental tugs before he pulled back and looked at his shirt binding his arms, his face scrunching up at the sight. "What?" He questioned, looking back to Sirius, suddenly feeling exposed.