The appearance of the unnamed passenger and the violent plans he details mark a turning point in the film, and after his ride with Travis, Travis's own violent plans begin to take shape. Halfway through the film, Travis pulls over to a curb and sits with the unnamed passenger, waiting quietly then listening as the passenger reveals his violent and sexually grotesque plans to murder his unfaithful wife. The passenger directs Travis's gaze, as a director might, to the lighted window where his wife is, tells Travis his wife is sleeping with a black man, then describes his plans for murder in gruesome detail. The passenger uses hateful language to describe the black man as well as what he'll do to his wife, giving voice to much of the hate Travis already feels. After this scene, nothing in the movie is the same. The passenger plants the idea of extreme violence in Travis's head. In just a few scenes, Travis will seek out a gun of his own, like the one the passenger has.

Scorsese makes a cameo appearance in Taxi Driver as the unnamed passenger, and this is not the first time in film history that a director has acted in the movie in a small role that changes the course of the film. In Chinatown, a 1974 film staring Jack Nicholson, the director, Roman Polanski, has a small but influential cameo as well. Halfway through the film, Polanski appears briefly as a thug who slashes the hero's nose, changing the film from a light detective story to a story that takes place in a dangerous underworld. Similarly, Scorsese's part in Taxi Driver broaches the idea of twisted violence, which had not been present before. Historical accounts of the making of Taxi Driver point out that another actor was slated to play the role of the passenger. When the actor got sick, Scorsese decided on the spur of the moment to play the role himself.