Director - Martin Scorsese

When screenwriter Paul Schrader wrote this scene it simply said "Travis talks to himself in the mirror" - there was no specific dialog given. Everything that insomnia-plagued taxi driver Travis Bickle (Robert De Niro) says during his faux-conversation was improvised by De Niro on the spot.
To this day, whenever someone walks by a mirror they can't help but utter his now famous line "You talking to me?"