There are many ad libs that actually made it into films.
What's your favorite line that was created by actors and not in the shooting script?
I'll go with a very obscure one. The movie is Home For The Holidays (1995), directed by Jodie Foster and starring Holly Hunter, Anne Bancroft, Claire Danes, Robert Downey, jr., Dylan McDermott, and Steve Guttenberg.
The line is by Robert Downey, jr, who ad-libbed a ton of material in the movie (according to Jodie Foster on the DVD commentary).
The line is:
"She feels her feelings, when she feels them."
The way he delivers the line, and the situation in which he delivers it, make it a hilarious ad-lib that Foster was wise enough to leave in.
I'd love to hear everyone's favorite ad-lib that made it into the final cut of a film.