Harry Caul
The tragically paranoid and reserved protagonist of the film. A staunch Catholic and a very private man, Harry works as a surveillance expert. He is serious about his job, and brings the techniques of surveillance into his private life, as well: has three locks on his apartment door, an extensive alarm system, and uses a payphone to make personal calls. Years ago, his surveillance work led to the murder of a family, an event which haunts him to this day. Because of his sense of guilt and his desire to behave ethically, he becomes obsessed with heading off what he fears will be the violent outcome of his current project, but soon finds himself in over his head. His only hobby is playing the saxophone along to jazz records in his home.
Stan
Stan works for Harry and has a much more casual approach to surveillance work. When they are spying on the couple in Union Square, Stan immediately becomes lasciviously distracted by a duo of young girls who approach the side of the van to look at their reflections. Later, he wants to speculate about the contents of the tapes, but Harry insists that they keep it professional, and scolds Stan for taking the lord's name in vain. Stan is light-hearted and likes to enjoy himself, which also makes him careless in Harry's eyes.
Ann
Ann is the wife of the director of the corporation. She is having an affair with Mark, who also works for the corporation, and Harry spies on them at the start of the film. Ann is depicted as empathetic, sweet, but troubled. By the end of the film, the viewer sees that Ann is not as innocent as she appeared.
Mark
Mark works for the corporation and is having an affair with Ann.
The Director
The director of the corporation, an eccentric and wealthy man who suspects his wife of having an affair. He has a faithful dog by his side and works in a large office in a high-rise office building.
Martin Stett
Martin is the director's assistant. He is a chilly, detached, and somewhat menacing presence in the film. While he behaves in a friendly manner towards Harry, he also becomes threatening once Harry refuses to hand over the tapes, warning Harry to "be careful," and following Harry to the convention.
Amy
Amy is Harry's mistress, who lives in a small apartment (paid for by Harry) all by herself. She sits in the apartment day and night, waiting for Harry to visit her. While she loves Harry and provides him with a respite from the stress of his job, she becomes impatient with Harry's withholding of information, and decides that she cannot wait for him any longer.
Bernie Moran
Bernie is Harry's professional rival and is depicted as almost his polar opposite. Where Harry is serious and concerned with duty, Bernie is sleazy and insinuating. While Bernie admits that Harry is a very good surveillance expert, he maintains a very arrogant attitude about his own work. He is depicted as a bon vivant with a lecherous sexual appetite and a tacky sense of humor. He desperately wants to go into business with Harry, but Harry refuses.
Meredith
Meredith is a lost woman who hangs around with Bernie and seduces Harry at the party at his office. She tells Harry that she worked her way up from being a secretary to becoming the boss's wife at a large company in New York, but has since left that position and moved. She proves to be a confidant and comfort for Harry, but she betrays him by stealing the tapes and giving them to the director.
Paul
Paul is a cop and friend of Harry's. He helps Harry collect the surveillance on Ann and Mark in Union Square, and later introduces Harry to Bernie at the convention.
