The slap (with sound effect which could easily be from Will's arm brushing against Chris' mic) could plausibly be fake but Chris' reaction right after did not look at all like it was "acting". Clearly, he was shaken. So, I'm leaning toward "not fake" but still confused and am really over this air of superiority this Hollywood "couple" prance around with.
Him winning the Oscar's definitely not gonna reign that in anytime soon, either.