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.
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 wookie back there on the expressway."
This film was made as a result of George Lucas's 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 "THX-1138", 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 re-bar embedded in concrete. The actors are actually crawling along a horizontal surface. By tilting the camera to appropriate angles, it appears that the characters are climbing upward.
There are a number of scenes which violate the "180° rule" in film making. This is intentional to convey a sense of disorientation and confusion which THX experiences now he has emerged from drug-induced sedation.
A scene in which THX falls into a garbage compactor and fends off a mutated rodent was cut because the monster did not look realistic. This situation was later reused in Star Wars: Episode IV - A New Hope (1977).
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 to improvise all the "overheard" dialogue in the movie.
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.
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".
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 near-fatal car crash as a youth.
Publicity photos and 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.
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.
George Lucas's 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.