Although we become well acquainted with Travis throughout Taxi Driver, his mental instability makes his actions unpredictable, and although Travis seems sympathetic, we never fully understand him. Travis is never part of the normal world. Though he initially wants to fit in and to be like other people, he is too mentally ill to act normally. Even at his best, at the beginning of the film, he can't sleep, drinks heavily, pops pills, and spends his mornings in porn theaters. After Betsy rejects him, Travis becomes hysterical, violent, and obsessive, and from here descends into madness. He loses all self-awareness and deludes himself into believing that shooting a presidential candidate and then shooting himself is a heroic gesture. Travis changes from a wounded man into a hardened one, testing our sympathies and distancing himself through violence. Young Iris prevents Travis from turning into a monster by giving him a reason to look at the world outside himself. Even as Travis plots his heroic act of violence, he worries about how to save Iris. He believes he has cut himself off from all worldly feelings and that he is just training to be a soldier, but his concern for Iris suggests otherwise. Travis's many contradictions make him one of the great characters in film history.
We never learn exactly what happened to Travis during Vietnam, and the rest of his past remains unexplored, so there's no way to explain why Travis has become the way he is. His war experiences must have influenced his character, acquainting him with violence and helping to turn him into a killer. Travis's anger wouldn't be so frightening if he wasn't able to transform himself into a warrior so efficiently. When Travis goes to kill Palantine, he sports a new Mohawk haircut. The 101st Airborne paratroopers made this a popular haircut for American soldiers to wear into combat when they flew in on D-Day in World War II, and Travis's Mohawk shows the influence of his experience in the army on his character. Travis has also been influenced by his parents and his upbringing, though we never catch any glimpses of this past. His obsession with and disgust for all things sexual are surely rooted in early experiences, and his many comments about destiny or being chosen by God suggest that he may have had a religious upbringing as Scorsese and Schrader did. We know little about Travis outside of his taxi, and he remains a mystery.
Iris sees her New York life as glamorous and independent, and although being a prostitute at such a young age is unequivocally damaging, she most likely views Travis's rescue of her as a mixed blessing. In New York, she believes people love and need her, even if those people are her pimp, Sport, and the johns that visit her. She takes pride in being streetwise, as we see during her breakfast with Travis when she asks if he is a "narc" and turns his questions back on him. Though she is willing to entertain the thought of leaving Sport, she is about as likely to go back to her parents on her own as Travis is. Instead, she dreams of moving to Vermont. Though Travis has propriety, morality, and age on his side to justify his actions, his murders would have been more heroic had we not seen Iris dancing with Sport. Though he is her pimp, Sport comes across as comforting and romantic, and Iris clearly cares for him, for better or worse. Travis can neither see nor hear Iris and Sport in this scene. If he had seen it, he may have realized that his so-called heroism is most likely the least heroic in the eyes of the person he saves. After the murders, we don't hear from Iris again. She has disappeared back to Pittsburgh, and the only remaining sign of her is a grateful note her father wrote to Travis.
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.