- Accidentally Correct Writing: On the DVD commentary, Francis Ford Coppola says he was shocked to learn that the film utilized the very same surveillance and wire-tapping equipment that members of the Nixon Administration used to spy on political opponents prior to the Watergate scandal. Coppola has said this is the reason the film gained part of the recognition it has received, but that this is entirely coincidental.
- Actor-Inspired Element: Harrison Ford's part was initially intended to be a small cameo, written as little more than an office assistant. Feeling that the character was one-dimensional, Ford decided to play him as gay, a risky choice in 1974, and personally purchased the loud green silk suit for $900 ($4,284.99 in 2015 dollars). Francis Ford Coppola was at first shocked by the outfit at rehearsals but, after discussing it with Ford, was so impressed with this interpretation that he expanded the role into a supporting character, gave the character a name (Martin Stett) and had production designer Dean Tavoularis create an office that reflected the character's orientation.
- Cast the Runner-Up: Harrison Ford was originally considered for the role of Mark.
- Deleted Role: Abe Vigoda as Harry's lawyer and Mackenzie Phillips as his teenaged niece. Based on the stills of the deleted scenes with Harry's neighbors, it looks like one of them was played by Howard Hesseman.
- Deleted Scenes: Francis Ford Coppola and Walter Murch left at least an hour of footage on the cutting room floor, which has been lost except for stills. Most significant was a subplot of Harry dealing with his neighbors, who complain about the building's plumbing problems, unaware that Harry actually owns the building. Other scenes feature Harry consulting his lawyer (played by Abe Vigoda) about the apartment situation, and Harry convincing his teenaged niece not to run away from home.
- Doing It for the Art: Francis Ford Coppola agreed to make The Godfather Part II in order to get this film made.
- Dueling Works: In theaters at the same time as another Paranoia Fuel classic, The Parallax View.
- Dyeing for Your Art: Gene Hackman grew a pathetic looking moustache for the film.
- Inspiration for the Work: Coppola has cited Blow Up as a key influence on his conceptualization of the film's themes, such as surveillance versus participation, and perception versus reality.
- The Other Marty: Timothy Carey was originally cast as Bernie Moran. Production was shut down until he was replaced with Allen Garfield.
- Playing Against Type:
- Harrison Ford going for a much less heroic & charming role and more of a creepy villainous one as Martin Stett, though this film did precede his more well known roles. He was actually still largely paying the bills as a carpenter at the time, and got the role by building a custom bookcase for Francis Ford Coppola.
- Hackman, too. Caul is exceptionally skilled in his field but is a far cry from the kind of badasses Hackman usually plays. Coppola noted in the DVD commentary that Hackman, generally gregarious and relaxed, had a lot of trouble playing the nervous, reclusive Caul.
- Method Acting: Gene Hackman learned how to play the saxophone for this film.
- Mid-Development Genre Shift: It was originally envisioned as a horror film and would have starred Marlon Brando.
- The Other Marty:
- Filming began with Timothy Carey as William P. "Bernie" Moran, but things didn't work out (he claimed he caught a cold and Coppola took the opportunity to fire him, while other reports have the famously difficult Carey abruptly quitting). Production halted until Allen Garfield was hired to take over.
- Behind-the-scenes example: Haskell Wexler was the original cinematographer but got fired after shooting the opening scene and replaced by Bill Butler. Again this caused production to shut down for a few days. Wexler's scenes are still in the film, but he didn't get a credit.
- Real-Life Relative:
- Serendipity Writes the Plot: Harry was to have been named Harry Call, but a typing error led to his being named Harry Caul and the name stuck because Coppola liked how the meaning of the word caul (a birth defect causing a membrane to surround the head) related to the character.
- Shrug of God: In the last scene where Harry tears apart his apartment, Francis Ford Coppola stated on the commentary that he has no idea where the bug is. Two ideas he mentioned were the saxophone strap, or that there was no bug and Harry was delusional. Surveillance technology expert Martin Kaiser, who also served as a technical consultant on the film, suggested that it could be built into the saxophone itself, much like Soviet gifts to foreign embassies at the time, but that his idea wasn't definitive.
- Typecasting: An interesting use of it for The Director. Since he only appears in one scene we don't get much of a chance to learn about him, but since Robert Duvall specializes in playing stern, temperamental authority figures, we immediately know what kind of person he is once we see his face.
- Write What You Know: Coppola considers this his most personal movie, largely because he incorporated so much of his personality into Harry Caul. Like Harry, Coppola suffered from polio as a child which temporarily left him paralyzed. He was also obsessed with gadgetry from an early age and bugged his parents' phone when a teenager. Harry's ardent Catholicism and prudishness tally with many descriptions of Coppola.
Trivia / The Conversation