Taxi Driver You Talkin me

In the scene, Travis Bickle (De Niro) is looking into a mirror at himself, imagining a confrontation which would give him a chance to draw his gun. I believe that scene was improvised by De He says the following line: You talkin to me You talkin to me You talkin to me Then who the hell else are you talkin to You talkin to me Well I m the only one here. Who the fuck do you think you re talking to