In the Dark Knight, the joker was meant to walk away while the hospital exploded. But instead he stopped walking and in a moment of improvisation, he began smiling and fidgeting with the detonator.
Casablanca (1942). The scene of Rick Blaine (Humphrey Bogart) putting Ilsa Lund (Ingrid Bergman) and Victor Laszlo (Paul Henreid) on a plane bound for America with the help of Captain Louis Renault (Claude Rains) is chock full of memorable lines. But the line listed as 5th in AFI's '100 Years...100 Movie Quotes' wasn’t even part of the original script. According to reports, Bogart said the phrase “Here’s looking at you kid” multiple times to Bergman while teaching her to play poker between takes, and they just used it in the film.
In Good Morning Vietnam (1987), none of Robin Williams' broadcasts had scripts written for them. He improvised almost every word.
“Gooooood morning, Vietnam! Hello campers, remember Monday is Malaria day! That’s right, time to take that big orange pill…” - Robin Williams.