4. Don Vito Slaps Johnny Fontane - The Godfather
One of the most memorable moments from The Godfather is when Vito Corleone (Marlon Brando) slaps Johnny Fontaine (Al Martino) around the face and tells him, "You can act like a man!" after he starts whinging about not getting a movie part that he desperately wants. This was all on Brando; the famously eccentric actor decided that he wanted to bring a little more life out in Martino's performance, and so completely out of the blue, slapped him around the face. Watch Martino's face carefully; he clearly doesn't know how to react, but well aware the cameras are rolling, at least maintains a measure of shocked composure that transpires quite hilariously into one of the movie's best scenes.