Was Keaton really laughing?

At the end of the scene where Woody's character comes back to the beach house after the date where he has the run-in with the biker, Diane Keaton's character laughs at Woody's comments describing his ordeal.
It is true to her character, she would be relieved at the disaster since she is now pretty much in love with him and was uncomfortable when her husband told her of the date, etc.
But her laughter seems so genuine, i wonder if it was real?
OK, in a few years she'd be an Oscar winning actress, but still...
Also, I wonder if that scene and those lines are in the play, which she would have performed with Allen many times a few years earlier. If so, I guess her laughter was just good acting. If not, there's the possibility she's really cracking up and Woody's closing line about his sex-life turning into the petrified forest may be an ad-lib. He does pause a moment in the delivery of it...