Co-writer Walter Murch has said in interviews that George Lucas never explained the origins of the character names THX, SEN, and LUH to him, but he believes that they are deliberate homonyms for sex, sin, and love, the three factors that set them apart from society.
To provide the large number of extras required, George Lucas contacted the Synanon drug rehabilitation facility. He found many recovering drug users who were required to be shaved bald for the drug program anyway. The Synanon Facility is mentioned in Philip K Dick's science fiction novel "Valis".
The final part of the chase where the motorcyclist crashes was one magnificent stunt with no dummies. Stuntman Duffy Hambleton rode his police motorcycle full speed into a fallen paint stand, flew over the handlebars, was hit by the airborne motorcycle, landed in the street on his back, and slammed into the crashed car. According to the film's commentary, everyone at the location was stunned, and immediately ran in to ensure Hamilton was alright. According to George Lucas, it turned out Hamilton was perfectly fine, apart from being angry with the people who had run into the shot to check on him. He was worried that they might have ruined the amazing stunt he had just performed by walking into frame.
Shortly after THX steals a police car, and shortly before his fellow escapee crashes the one he tries to steal, you can hear someone on the radio say, "I think I ran over a wookiee back there on the expressway."
This film was made as a result of George Lucas' student film short project at USC, Electronic Labyrinth THX 1138 4EB (1967). Having won significant praise and attention for what was, at the time, an unconventional short, Lucas was given the opportunity to direct a feature-length version starring Robert Duvall, produced by his mentor Francis Ford Coppola under his newly formed production company American Zoetrope. Zoetrope was a financial failure, as was this movie, but the attention was enough to win Lucas the opportunity to make American Graffiti (1973), the success of which paved the way for the opportunity to make Star Wars: Episode IV - A New Hope (1977).
For the final sequence, in which THX is climbing up to the surface, it is a simple perspective trick. It is not a ladder, but rebar embedded in concrete. The cast members are crawling along a horizontal surface. By tilting the camera to appropriate angles, it appears that the characters are climbing upward.
There are several scenes which violate the "180° rule" in filmmaking. This is intentional to convey a sense of disorientation and confusion which THX experiences, now he has emerged from drug-induced sedation.
To provide the voices of the unseen overseers and announcers, George Lucas contacted San Francisco-based theater group The Committee. He gave them brief character outlines, and allowed the actors and actresses to improvise all of the "overheard" dialogue in the movie.
George Lucas has worked the title of this film, or parts of it, in some of his other films. In American Graffiti (1973), the license plate of one car is "THX 138". In Star Wars: Episode IV - A New Hope (1977), a reference is made to "prison cell 1138". The cinema sound certification his company developed is called "THX". In Star Wars: Episode I - The Phantom Menace (1999), one of the battle droid's number is "1138". It is seen when all of the battle droids have been shut down.
A scene in which THX falls into a garbage compactor and fends off a mutated rodent was cut, because the creature did not look realistic. This situation was later re-used in Star Wars: Episode IV - A New Hope (1977).
Though it's obvious that THX works at a factory that makes his society's android police officers, the strange levers and thickly sealed glass screen he stands in front of at his job should be familiar to those who work in nuclear engineering facilities. George Lucas shot the scenes of THX at work in front of a functioning radiation containment chamber, also known as a "hot cell", which was used by those who must handle radioactive materials, like isotopes.
George Lucas claims that the scene where technicians mess with THX's nervous system, sending him into comical spasms, was drawn from his antipathy towards the doctors who treated him after his nearly fatal car crash as a youth.
George Lucas adapted the idea for the original short, and eventually this movie, from a one and a half page outline called "Breakout", which was written by fellow USC student Matthew Robbins, about a man escaping an underground dystopian society.
In the scene featuring fetuses in jars, and a rotating machine, Sound Designer Walter Murch quoted a piece of musique concrète (a squeak from a barn door in Normandy) by Pierre Henry as an homage. After the film ran theatrically in France, Warner Brothers received a letter from Henry's record company claiming breach of copyright. It was eventually ruled that because the squeak had been extensively modified, and that it was a very brief quotation, it did not constitute an infringement.
On completion of editing of the film, Francis Ford Coppola took it to Warner Brothers. Executives there disliked the film, and insisted that Coppola turn over the negative to an in-house Warner Brothers editor, who cut approximately four minutes of the film prior to release.
George Lucas attempted to tell a story that critiqued how the particular political atmosphere of consumerism and conformity of the era, in which the movie was made, could lead to a dystopian future. As such, SEN, the member of society who attempts to pressure THX to conform, was given dialogue culled from speeches by President Richard Nixon, including his "We need dissent, but creative dissent!" ramblings.
During his arrest, THX's work location is identified as "Operating Cell 94107", referring to the zip code of San Francisco's SOMA (South of Market) district where American Zoetrope's offices were located at the time. By interesting coincidence, Soma is the name of a narcotic from Aldous Huxley's "Brave New World".
Studio sequences were shot at stages in Los Angeles, including a white stage one hundred feet (thirty and a half meters) long by one hundred fifty feet (forty-six meters) wide for the "white limbo" sequences.
George Lucas' original plan was to shoot the film in Japan, but Francis Ford Coppola did not give Lucas enough money in the film's budget to take the entire production to Japan. The film was shot in San Francisco and Los Angeles.
Publicity photos, some foreign posters, and video covers feature a shot from a scene not included in the final film: The police robots approaching the dead body of the OHM priest (who SEN killed earlier) and checking for a pulse.
George Lucas included the trailer to "Tragedy on Saturn, Chapter Two of Buck Rogers", a 1939 Saturday morning serial, before the film. He attempted to draw an ironic contrast between the swashbuckling Rogers and the title character in his bleak science fiction debut, because each was "just an ordinary, normal human being who keeps his wits about him." But he also saw it as an homage to the kinds of science fiction stories he loved growing up. The serialized space adventures of Buck Rogers were also a fundamental influence on Star Wars: Episode IV - A New Hope (1977).
After completion of photography, Francis Ford Coppola scheduled a year for George Lucas to complete post-production. Lucas edited the film on a German-made K-E-M flatbed editor in his Mill Valley house by day, with Walter Murch editing sound at night. The two would compare notes when they changed over. Murch compiled and synchronized the sound montage, which includes all of the "overhead" voices heard throughout the film, radio chatter, announcements, et cetera. The bulk of the editing was finished by mid 1970.
Some producers felt that this was the kind of film where the special effects or drama should be right at the start, instead of a source of suspense. In this case, it was the scene where THX was attacked by shell-dwellers. The producers wanted to "get the freaks up front".
The Lavender Hill Mob (1951) featured a car chase where Holland, the protagonist mentions a license plate as THX 375. The cinematographer on this film was Douglas Slocombe, who worked in three films written and produced by George Lucas: Raiders of the Lost Ark (1981), Indiana Jones and the Temple of Doom (1984), and Indiana Jones and the Last Crusade (1989).
The trivia items below may give away important plot points.
The seemingly endless Control Room where the android police try to corner THX and SRT, who find out LUH has been consumed for organ reclamation, was the circuit switch room of the San Francisco location of the Pacific Bell Telephone Company. Pacific Bell allowed George Lucas to shoot the film there, because the entire room and the hardware found there were about to be dismantled, as the phone company was switching to touchtone phone technology.
Following the huge success of George Lucas' Star Wars: Episode IV - A New Hope (1977), Warner Brothers decided to give the original, unaltered cut of this movie a limited release in 1978, and this was the version that came to home video in the early 1980s. The trailer featured the tagline, "Before George Lucas explored the outer regions of 'Star Wars', he explored the inner regions of "THX 1138." The film's novelization by Ben Bova was also reissued to coincide with the re-release of the film. Although the original 1971 edition of the book left THX's ultimate fate unknown, ending just as he's about to climb to the surface, the 1978 edition features an extended ending in which he emerges and discovers a beautiful (and populated) world outside.
The end shot of THX escaping and seeing the setting sun for the first time wasn't a special effect, or timelapse shot. Uncredited cameraman Caleb Deschanel and Matthew Robbins scouted a location for George Lucas, and found a perfectly clear horizon for the shot in Port Hueneme, California. Robbins and Deschanel tried to get the shot four times, but the weather made it impossible until it was captured on the fifth attempt.