Uncontrollable laughter — a case of the giggles — sometimes just happens on set. It’s often triggered by one line of ordinary dialogue, that the actor just can’t say with a straight face. The more he tries not to laugh, the more he laughs.
This inability of the professional actor to be “professional”, his helpless mirth, reveals his humanity, and his infectious laughter clears the set of bad vibes and does everyone a world of good.
This video of Brad Pitt with the giggles is from the dailies of Moneyballs. His laughter is made worse when his off-screen co-star Jonah Hill offers earnest acting advice: “Use it, Brad”.