Strange has his nose in a book so he doesn't really notice what Joker's doing until the other man walks behind him. Strange turns around right in time to see Joker pantomime hitting him over the head.

Surprising no one, he doesn't find this the slightest bit funny.

"We are in a library!" he hisses under his breath, giving the angel a glare. "If you're going to roughhouse, do it outside."