Does Al Pacino really slap Diane Keaton in The Godfather II?

This might be second-hand and I can't find a video, but it seems legit*: On May 1, 2014, Keaton read from her book Let’s Just Say It Wasn’t Pretty at the 92nd Street Y community center and answered some questions from the audience. And when the subject came up, she confirmed the slap was real:

She loved getting slapped around by Al Pacino in The Godfather. Remember that scene in Part II?

“He really slapped me! It was amazing and it really worked. I loved it.”

*[Seeing that it's an official recap of the event by the 92Y organization, there's no reason to assume that the information is disingenuous.]

I think the slap was fake, also just as the blows that were given to Carlo were fake, you could see it. Sonny did a good act, but they were still fake; I think the garbage can was also not a real one but a softer prop.

Yes it was a real slap. It was the perfect reaction from a real man and a gangster who a woman had just told she aborted his baby. Corleone family as shown through out the whole trilogy is quite devoted towards family traditions, the slap was out of desperation.

There is no no quotes from Keaton on whether it was authentic but this passage from Deborah C. Micthell’s biography “Diane Keaton: Artist And Icon” is pretty noteworthy: “When her parents saw Godfather II in Los Angeles, the audience applauded when Michael slapped Kay in the confrontation scene. She (Keaton) defensively explained: ‘he was a horrible character…I say to Hell with those people who applaud. My parents were with me.

This kind of reaction can only come from someone who really experienced it. I hope it answers your question.