The role of Gort was played by Lock Martin, the doorman from Grauman's Chinese Theater, because he was extremely tall. However, he was unable to pick up Helen because he was so weak and had to be aided by wires (in shots from the back where he's carrying her, it's actually a lightweight dummy in his arms). He also had difficulty with the heavy Gort suit and could only stay in it for about a half hour at a time.
To give the appearance of seamlessness to the space ship, the crack around the door was filled with putty, then painted over. When the door opened the putty was torn apart, making the door seem to simply appear.
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, and she found it difficult to keep a straight face while saying her lines.
The phrase "Klaatu barada nikto" has become a popular phrase among sci-fi fans over the years and has been featured in other movies, such as Army of Darkness (1992). The line was also used in an episode of "The Rockford Files". Jim Rockford says it to a huge henchman of the episode's "bad guy".
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.
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. Studio head Darryl F. Zanuck had shown the script to Spencer Tracy, who was eager to play the role. Producer Julian Blaustein objected, saying that the audience would have numerous expectations about the character upon seeing an actor of such repute emerging from the flying saucer. Blausteinknew that Zanuck had the ultimate control, and if he insisted, Blaustein would either have to resign, or make the movie in an unsatisfactory way. Fortunately, Zanuck agreed, and Rennie was cast instead.
The scene of the large crowd fleeing the saucer area after Gort appears is all too obviously "sped up" film, making the shot look unnatural. The reason for the sped up film effect was explained by director Robert Wise in an interview. It seems that, despite much pleading and cajoling from him, the crowd of inexperienced extras portraying the saucer onlookers simply wouldn't move away from the saucer quickly enough to look panicky and convincing. After several takes, Wise finally had to move on with filming and reluctantly allowed the scene to be "sped up" in post production, knowing that the end result would probably look strange.
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.
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.
Whether the makers of the movie intended it or not, there is a striking resemblance between Klaatu and the head of the Manhattan Project, Robert Oppenheimer. Oppenheimer was known to make corrections on the blackboards of theoreticians at the Project, similar to the way Klaatu corrects the work of Professor Barnhardt. Oppenheimer's other-worldly brilliance and association with destructive power that could threaten the existence of the world seem like more than a coincidence.
According to the shooting script on the DVD special features, significant dialogue between Klaatu and Helen Benson was cut. That dialogue makes it clear that they have developed warm feelings for each other, a bond that is closer than the friendship they have in the final cut of the movie, although it remains unfulfilled. When Klaatu tells her that he and Gort will be leaving soon, she tells him how much she and Bobby will miss him. The thought of leaving her and Bobby behind is equally difficult for Klaatu.
People have found fault with the concept that no one knows what Klaatu looks like, since all of his doctors, nurses and Mr. Harley from the White House have seen him in the hospital, and would be able to recognize him easily. In fact, there was a scene which was cut, in which Klaatu is taken to a police station along with other men who were suspected of being the "space man". Klaatu admits that he has no identification, and is nearly taken to another area where eyewitnesses would be able to identify him. Fortunately for him, the military intervenes because Professor Barnhardt is asking to see "Mr. Carpenter", and Barnhardt's high standing with the government trumps the police department's procedures. This explains why Klaatu is taken from the boarding house by a government agent (Mr. Brady) who works with the police, but is then seen entering Barnhardt's home accompanied by an army captain. In reality, the cut scene would have tied all of this together, but director Robert Wise felt that it disrupted the flow of the film, and that the audience wanted to see the meeting between Klaatu and Barnhardt more than they needed to see the scene in which Klaatu narrowly avoids being identified.
In the opening title montage of astro-photographs representing Klaatu's trip to Earth, the last object seen before Earth and Moon come into view is the Eagle Nebula, Messier 16 in the constellation Serpens, and is centered on "The Pillars of Creation", an object that was later captured by the Hubble Space Telescope in an iconic image that became synonymous with the resolving power of this telescope.
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.
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.
Some reference works state that Adventures of Superman (1952) star George Reeves appeared as a television news reporter with eyeglasses in one sequence. This is not true. The actor playing the role bears no resemblance to Reeves, and in a 1995 interview with Reeves biographer Jim Beaver, director Robert Wise stated unequivocally that it is not Reeves in the role. It appears that someone jumped to conclusions based on the image of a reporter wearing glasses and thus resembling roughly the image of Superman alter-ego Clark Kent. Reeves had nothing to do with the film in any capacity.
In addition to the two Gort costumes worn by Lock Martin, a fiberglass statue of the robot was also made. This was used during the close-ups on Gort when he was firing his energy beam weapon and in scenes when he was not required to move.
One of the more distinctive, and subliminally eerie, effects in the film is the director's use of a musical chord, comprised of two different notes on an organ, which is mixed in with the engine and wind sounds of Klaatu's saucer as it flies over the D.C. area at the beginning of the story.
Lock Martin (Joseph Lockard Martin) was perhaps "discovered" working as a doorman, but this film was definitely not his first: his first movie role was a credited supporting (with lines and a close-up!) Sultan's guard in "Lost in a Harem" (1944). A few more movies and a theatrical comedy duo (with a midget) traveling with Spike Jones's stage show followed. Then, he helped create the immortal Gort.
At the boarding house, Mrs. Barley is played by Frances Bavier and Mr. Krull is played by Olan Soule. The two actors would team up again about ten years later on The Andy Griffith Show (1960): Bavier as Andy's Aunt Bee and Soule as John Masters, the Mayberry choir director.
The trivia items below may give away important plot points.
Originally Klaatu's post-death resurrection at the end of the movie was meant to be permanent, reinforcing his God-like powers. But at the time, the Breen Office -- the film industry's censors -- didn't like the ending, suggesting it was too left-wing, and insisted that director Robert Wise and writer Edmund H. North put in the line, "That power is reserved for the Almighty Spirit". Both Wise and North hated the line and thought it completely inappropriate -- negating the concept of Klaatu's race being all-knowing and all-powerful -- but the studio wouldn't back them up, and they were forced to put it in.
Klaatu establishes that he traveled to planet Earth from 250 million miles away. 250 million miles is equivalent to 402 million kilometers and 0.00004 light-years. It would locate Klaatu's home in a point between Mars and Jupiter, inside the known Solar system.
There are parallels between Klaatu and Jesus Christ. Klaatu comes from another world and chooses the name of Mr. Carpenter (the occupation of Jesus Christ). He came to warn about the destruction Earth is going to face if they don't believe him and his words. Jesus Christ said to the people that they will face destruction if they don't believe in him and in his words. Like Jesus prophesying about the destruction of Jerusalem, Klaatu prophesies that Earth may have to face destruction like leveling New York City or sinking the Rock of Gibraltar. Both Jesus Christ and Klaatu die and later come back to life. Like Klaatu's flying saucer seen by people in the film, people in Jerusalem and in Judea saw flying chariots and soldiers in the sky right before the destruction of Jerusalem in 70 AD. This was recorded by Hebrew Historian Josephus, Roman Historian Tacitus, Eusebius, Document "Pseudo Hegesippus", and Jewish History Document "Sepher Yosippon".
In the original script, Klaatu's resurrection scene was to have taken place in the space craft's medical lab, not the main control room. The manner in which Gort revives Klaatu was also written completely differently.
In the scene where Gort is seen carrying Klaatu's body (inside the ship), Michael Rennie was actually sitting on a dolly that is unseen by the camera, since Lock Martin (Gort) was unable to support Rennie's weight himself.