Taxi Driver  

In the 1976 classic Taxi Driver, Travis Bickle, played by Robert De Niro, was talking to himself in the mirror in his apartment to amp himself up to help someone. Bickle ends up becoming more of a villain than hero, but this scene exemplifies his misplaced confidence. 

The script of the movie simply stated that he was supposed to talk to himself. But Robert De Niro being the actor that he was did turned that note into gold. He ended up repeating the now iconic words that people have said into mirrors since then: “You talkin’ to me?”