"Oh!" Aziraphale sputters for a moment of utter bafflement when Crowley actually punches Spike in the face - over what, even? Crowley's called him variations on 'nerdy arsehole', and plenty worse, a thousand times before. And although his instant reaction should be to scold the demon and see to Spike's wellbeing - though an undead semi-immortal demon-type person, he is a person, after all - he instead feels his confusion resolve quickly into an acute sort of pleasure, like, er, well... what is he, even? Exhilarated? Flattered? Chuffed?

Crowley recoils in pain, and Aziraphale is at his side in an instant, pawing worriedly at him. "Are you all right?" he asks, idiotically, HE'S the one who started it!! but he can't help it. "What - what's gotten into you?"

Either of them, really, but let's start here.