Gene Hackman's character 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..
Gene Hackman was a fit, good-looking relatively young man when Francis Ford Coppola cast him as Harry Caul. In order to personify Harry's weary, aging, and unhappy existence, Hackman grew a pathetic-looking mustache, wore ill-fitting glasses, and had a wardrobe picked out that was at least 10 years out of date. Coppola specifically told Hackman he wanted Harry to look like a "nudnik", a Yiddish word referring to a person who is boring and a pest.
The blue Mercedes limousine that Cindy Williams is sitting in near the end of the film was won by Francis Ford Coppola on a bet with Paramount Pictures. Coppola had complained about the station wagon he shared with five other passengers during the filming of The Godfather (1972) and studio execs told him if Godfather grossed a certain amount they would spring for a new car. After Godfather was a huge hit, Coppola and George Lucas went to a dealer and picked out the Mercedes, telling the salesman to bill Paramount.
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.
In the original script, Harry Caul was the owner of the building in which he lived. There was a deleted scene where he had a meeting with the other tenants. One of the people there was Mrs. Evangelista. Now, we only know of her character when Caul speaks to her on the phone after she leaves him a birthday present.
Francis Ford Coppola noted in the DVD commentary that Hackman had a very difficult time adapting to the Harry Caul character because it was so much unlike himself. Coppola says that Hackman was at the time an outgoing and approachable person who preferred casual clothes, whereas Caul was meant to be a socially awkward loner who wore a rain coat and out-of-style glasses. Coppola said that Hackman's efforts to tap into the character made the actor moody and irritable on-set but otherwise Coppola got along well with his leading man.
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. Not only was the script for The Conversation completed in the mid-1960s (before the Nixon Administration came to power) but the spying equipment used in the film was discovered through research and the use of technical advisers and not, as many believed, by revelatory newspaper stories about the Watergate break-in. Coppola also noted that filming had been completed several months before the most revelatory Watergate stories broke in the press. Since the film was released to theaters just a few months before Richard Nixon resigned as President, Coppola felt that audiences interpreted the film to be a reaction to both the Watergate scandal and its fall-out.
Due to creative differences on this shoot, veteran cinematographer Haskell Wexler was replaced by DP Bill Butler. According to Coppola, Wexler visualized the movie in the more romantic style of The Thomas Crown Affair (1968) while Coppola saw it more in the cinéma vérité style of Medium Cool (1969) (Wexler was cinematographer on the former and directed the latter).
The original cut was four and a half hours long. 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 (played by Mackenzie Phillips) not to run away from home.
The meaning of Harry's last name, Caul, is a fetal membrane sometimes present at birth. This ties in strongly with both Harry's transparent rain jacket, which he wears for the majority of the film, and also the fact that Harry is occasionally viewed through a translucent sheet of plastic when threatened, such as by his rival during the party scene.
During the party in the warehouse, Bernie Moran brags that 12 years before he had recorded the calls of an unnamed Presidential candidate and may have determined who won the election. This presumably would have been the 1960 election, when John F. Kennedy narrowly won the election over Richard Nixon, who was at the time of the movie's production in the middle of his own taping scandal known as Watergate.
Some fans believe that Gene Hackman's character in Enemy of the State (1998) is the same man who has over-reacted to the events that conclude this film and has gone "off the grid" under this identity. There is no evidence this is true.
The trivia items below may give away important plot points.
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.
Francis Ford Coppola: When Harry Caul turns on the TV in the Jack Tar Hotel to blot out the sound of the murder in the neighboring room, the broadcaster is Coppola talking about Richard Nixon and the Watergate scandal - itself triggered by a notorious piece of bugging.
The Jack Tar Hotel on Van Ness Avenue, site of the grisly murder scene, opened in 1960 and later became the Cathedral Hill Hotel in 1982. It was demolished in 2013, to be replaced by a hospital complex.
According to Martin Kaiser, the final scene of the film - in which Caul is convinced he is being eavesdropped in his apartment, cannot find the listening device, and consoles himself by playing his saxophone - was inspired by the passive covert listening devices created by Léon Theremin, such as the Great Seal bug. "He couldn't find out where [the bug] was because it was the instrument itself."