His heart is pounding in his chest, which means he either isn't faking this response or is very, very good at doing it.

"I - that wasn't - I didn't - "

You could try to be suavely mysterious. I'm working so hard on your behalf.

Steve will have absolutely no context for the strangled sound of sheer, indignant annoyance. That means Zerxus can't even focus on shouting at the devil - not because it's an extremely stupid thing to do, because that's never stopped him before, but because it leaves Steve in the dark, maybe thinking it's something he did.

"I'm sorry. He's playing with you." And Zerxus, obviously, but that much is a given.