"Lux Radio Theater" broadcast a 60 minute radio adaptation of the movie on January 4, 1954 with Michael Rennie reprising his film role.
Robert Wise was attracted to the project because of its overt anti-military stance and also because he believed in UFOs.
Darryl F. Zanuck was the one who first suggested Michael Rennie for the part of Klaatu, having seen him perform on the London stage.
Bernard Herrmann used two Theremins to create his creepy score, one pitched higher, the other lower, making this one of the first films to feature a largely electronic score.
Bernard Herrmann's music for the film is scored for two theremins, pianos, harps, different electrical organs, percussion, amplified solo strings and a large brass section including four tubas.
Patricia Neal has admitted in interviews that she was completely unaware during the filming that the film would turn out so well and become one of the great science-fiction classics of all time. She assumed it would be just another one of the then-current and rather trashy flying saucer films that were popular at the time, and she found it difficult to keep a straight face while saying her lines.
Harry Bates was paid a mere $500 by 20th Century-Fox for the rights to his short story "Farewell to the Master".
All of the scenes of Helen Benson and Klaatu in the taxi also feature footage from the second unit of Washington, DC as we see background vehicles in the rear and side windows of the taxi.
Although he was already signed to play the Einstein-like Professor Barnhardt, the studio wanted to remove Sam Jaffe as a result of the political witch hunts that were then underway. Producer Julian Blaustein appealed to studio chief Darryl F. Zanuck. Zanuck allowed Jaffe to play the role, but it would be Jaffe's last Hollywood film until the late 1950s.
As an homage to this film George Lucas named three of the alien bounty hunters in his Star Wars trilogy "Klaatu", "Barada" and "Nikto".
Because the stationary Gort could not stand on the angled ramp, Lock Martin had to wear the Gort suit in the background during the final sequence. Martin, who was frail, had to wear the suit for so long that he began having spasms in his arms. During Klaatu's final speech, Gort's arms can be seen moving slightly.
Doubles were used for Klaatu and Bobby in long shots of them walking around Washington, DC. In reality, none of the principal cast ever went to Washington, and the scenes with Klaatu and Bobby at the Lincoln Memorial and at Arlington Cemetery were shot in front of background screens using footage shot by the second unit crew in Washington, DC.
During the early phases of pre-production for The Day the Earth Stood Still, 20th Century-Fox studio chief Darryl F. Zanuck suggested Jack Palance for the role of the robot Gort. The role was eventually filled by a much taller non-actor.
In 1951, 20th Century Fox theatrically distributed this with the short film The Guest.
In line with the film's Christian allegory, Klaatu adopts the name "Carpenter" when hiding out from the authorities. Robert Wise hadn't considered the Christian implications until it was pointed out to him several years later.
In the original story, "Farewell to the Master", the robot's name was Gnut, not Gort.
In the original story, the robot, Gort, was the master - Klaatu was merely one of a series of doubles, or maybe clones, that died after a short time.
In the scene where Gort is seen carrying Klaatu's body (inside the ship), Michael Rennie was actually sitting on a dolly out of camera angle to support his weight during this brief scene, since Lock Martin (Gort) was unable to do so himself.
In the scenes of Gort carrying both Helen Benson and Klaatu up the ramp and into the ship, lightweight look-alike dummies were used because of Lock Martin's inability to actually carry either actor himself.
One of the reasons that Michael Rennie was cast as Klaatu was because he was generally unknown to American audiences, and would be more readily accepted as an "alien" than a more recognizable actor.