I agree their pros, actors, this is a nothing burger performance.
Also agree Rock looked like he was anticipating the slap. Didn’t even raise hands in defense and turned his head. It was quick, though, and the head turn could’ve been involuntary.
But not convinced one way or the other.
And he immediately continued with “oh my, I was slapped by...”.
Even when Will got up he said “ uh oh, Wills getting up”.
Staged.
If you watch Rock’s facial movements comparing pre/post slap, it looks like the side of his face was numbed, demonstrable in his speaking after the slap as well. If he was faking, that was some class A acting.
Look where Rock’s eyes are focused... appears to be Smith’s left hand. As that hand comes up, Rock rolls his head away from Smith’s right-cross. A tad late as Smith partially connected with his finger tips. Rock isn’t trained in studio fights, but he did a pretty good job in his first outing.
Smith tips the punch/slap a la professional wrestling.
They’re their there
I think Rock flinched because he could see something coming. He probably didn't know what it was going to be, but it is natural to flinch under the circumstances.