The Godfather-1972No one knows where the hell Brando got the cat come from. It was never in the script.
Tommy Lee Jones famous answer to Harrison Ford's plea in the sewers that he didn't kill his wife was simple but perfect, and entirely ad-libbed. "I don't care."
The Dark Knight Rises-2008
The slow clap of a creepy clown was never meant to be but only the dark brilliance of Heath Ledger could make it so.
Snapping closed the lid of the jewelry box was a playful spontaneous move by Richard Gere but Julia Robert's reaction was so perfectly honest, they decided to keep the scene.
To see a much larger and mind blowing list hop on over to Screen Rant.
And remember...life is meant to be ad-libbed.