Based on the body language of both Will Smith and Chris Rock, Darren Stanton believe the slap was 100% real.
Stanton is an expert in reading body language. He's so good people have dubbed him the "human lie detector" and he's claiming that that Will Smith/Chris Rock confrontation was not staged, as many are believing. Even with the expert level actor that did it, He says there’s no way it was faked. He doesn't see how Smith could fake “that level and visible shift of emotion.” Stanton also said that watching how Rock carried himself, he wasn’t expecting what happened.
If you watch the recovery back to the script, he seems disoriented a little and even stumbles over what he's reading on the prompter.
Stanton said, “Once Chris Rock made the joke about Will Smith’s wife Jada Pinkett Smith, you could see a visible emotional shift within the couple." Jada was "clearly uncomfortable and she showed signs in her face of awkwardness and disdain after being embarrassed in front of her peers and the world."
As far as Will laughing, then getting mad. He said, "Will made a slightly loose laugh and leant back in his chair, which is a sign of tension or uneasiness."
Many people think Rock's stance looked like he was bracing for the slap, but Stanton said Rock's stance, "with his hands behind his back, he displayed a position of authority, like many police officers, politicians and other empowered people stand in certain situations. He was also exposing most of his body and vital organs, a sign that there didn’t seem to be any imminent danger."
Source: LAD Bible