Sirius shouting "LUPIN," every time Remus walks in the room and "LUPOUT," every time Remus leaves.

Lupout, remus whispers to himself with a smirk on his face, his last word before he's reunited with his dear friend once more.

After he dies and climbs aboard the train, he walks down to the compartment where he and the lads used to sit. When he opens the door he's looking at his feet cause he's just so confused about what the hell is going on but then

"Lupin"

He looks to that young, rebellious boy smirking mischievously at him and says "what do I have to do for you to stop this joke?" Sirius just sighs, smiles brighter then ever and says "its so obvious, isn't it? Who would've said, a smart guy like you took the entire life to understand?

Just never leave"