So, the question in my mind would be, at what point defending yourself or your loved one from an insult/joke would get you to act this way? If somebody punches me, I would act back, but when it comes to a bad joke or comment, I'm not sure.
Hard to know what one would do without being in such a situation since much of what we do depends on context.
I have a job where being insulted is a very frequent occurrence and I've developed a thick skin such that most insults just roll off me like water off a duck's back. However, I do have a line when it comes to jokes and insults. Cross the line and I will push back in one way or another. Violence isn't my go to for most situations, but in some situations it's not off the table. As with all things, the devil is in the details.
As for the slap itself, from my experience in martial arts where I've been slapped a few times both on purpose and on accident as part of training, Wing Chun has a technique called Pak Sao or 'slapping hand' that can be used offensively or defensively, as well as my brief and limited experience with stage acting the slap looks real to me. A poorly executed one but real nonetheless.
I'll give my observations of the three main things that lead me to this conclusion.
First, Smith's body posture and movements. The way he turns with his hips into the slap. The way he comes up on his feet and leans back with his head while bringing his non-slapping hand up to protect his face. His tense and rigid posture during the slap. These are all indicators to me that Smith really hit Rock. It ticks all the same boxes of street fights I've watched where the attacker is rigid and awkward because they've not been trained on how to relax the body and plant oneself for a solid hit. The adrenaline shuts down fine motor movement so all of one's movements become more awkward. Which is why Smith stumbles a bit after the hit. Again, it looks like a shoddy slap from a trained eye but still real.
Second, Rock's reaction. His cranial velocity accelerates after impact. It's not like one sees in a stage slap where the head turns at the same speed. There's a clear increase in speed, or a whip of the head, after the hit. He's also leaned forward right up until Smith's hand is coming towards him and only then does he start to pull back as a protective reflex. But note that it's not a turning away as in a stage slap but his whole body is tensing in preparation for impact. If this was him acting it was the single best act he's ever performed in his entire career because that is exactly what people who're about to get hit do. Again, this coming from my experience both hitting and getting hit by people.
Third, black-on-black violence. These people are supposed to be woke AF. Why would Smith agree to portray himself as a violent black man which then perpetuates the stereotype that black men are violent? Why would the Academy agree to the same? It makes no sense from a woke standpoint. Surely they could've come up with a more woke friendly way to get attention if that was their plan.
Those were my main observations, but there's also this guy who is an actor and director who picked up on some of the things I didn't know how to evaluate such as the story aspect. If it was an act then there was a story being told where Smith was playing the part of the heel (as in the wrestling term) and that story doesn't make sense. Here's the video:
Edit: Forgot to mention that when I first saw the video it struck me as a manifestation of societal splintering and the mental breaking of people due to the intensity of pressure that's been building these last few years.
It's possible that in addition to that generalized pressure Smith feels a type of cuckolded rage at his wife sleeping with other men even if he says he's okay with it. If he saw his wife's look after the joke or she gave him a look that was similar and he felt that cuckolded rage, that would certainly explain his over-reaction.
He's also since come out and apologized for the slap. One could read that cynically as further legitimizing an event that was fake, but it's also possible that he's actually apologizing for something he really did. Hard to know for certain but it would be what one would expect in the case that it was real.