Robert De Niro worked fifteen hour days for a month driving cabs as preparation for this role. He also studied mental illness.
Director Martin Scorsese claims that the most important shot in the movie is when Bickle is on the phone trying to get another date with Betsy. The camera moves to the side slowly and pans down the long, empty hallway next to Bickle, as if to suggest that the phone conversation is too painful and pathetic to bear.
The scene where Travis Bickle is talking to himself in the mirror was completely ad-libbed by Robert De Niro. The screenplay details just said, "Travis looks in the mirror." Martin Scorsese claims that he got the inspiration for the scene from Marlon Brando mouthing words in front of a mirror in Reflections in a Golden Eye.
Harvey Keitel rehearsed with actual pimps to prepare for his role. The scene where his character and Iris dance is improvised, and is one of only two scenes in the film that don't focus on Bickle.