Dirty Dancing (1987) - directed by Emile Ardolino. Starring Jennifer Grey, Patrick Swayze and Jerry Orbach.
The crawling dance that is one of the best-known scenes in the movie—the rehearsal sequence set to Love Is Strange—was the product of director Emile Ardolino’s fondness for ad-libbing. Swayze and Grey got on the floor of the dance studio during a warm-up, and Ardolino asked them to do it again for the camera (x).
Roman Holiday (1953) - directed by William Wyler. Starring Audrey Hepburn, Gregory Peck and Eddie Albert.
For the famous “Mouth of Truth” scene, Gregory Peck ad-libbed the joke where he pretends that his hand was bitten off in the mouth of the stone carving. He borrowed the gag from Red Skelton. Prior to filming the scene, Peck told director William Wyler that he was going to do the gag, but that they should not tell Audrey Hepburn. When Peck pulled his arm out of the stone carving’s mouth with his hand pulled up his sleeve, Hepburn’s horror and surprise was genuine. She gave what she later recalled was “a good and proper scream,” and the scene was finished in one take (x).
Taxi Driver (1976) - directed by Martin Scorsese. Starring Robert De Niro, Jodie Foster and Cybill Shepherd.
2001: A Space Odyssey (1968) - directed by Stanley Kubrick. Starring Keir Dullea, Gary Lockwood and William Sylvester.