Star Trek (1966) creator Gene Roddenberry has been quoted as saying that this film was a major inspiration for that series. Perhaps not accidentally, Warren Stevens, who plays "Doc" here, would later be a guest star in 1968's Star Trek: By Any Other Name (1968), where the true shape of the alien Kelvans, like the Krell in this movie, was implied to be extremely non-humanoid but never shown. 1701, which is the serial number of the Starship Enterprise, allegedly comes from the clock mark 17:01 when the C57D enters orbit around Altair IV.
Louis Barron and Bebe Barron worked on the electronic soundtrack music "tonalities" for only three months, the length of time given them by Dore Schary, head of MGM. He authorized the studio to send them a complete workprint at Christmas 1955. They received the complete 35mm Eastmancolor workprint at New Year's 1956, a week later, still with many visual effects sequences missing and timed in with blank leader by editor Ferris Webster. From January 1, 1956 to April 1, 1956, they worked on the soundtrack score in their Greenwich Village studio in New York City while the film was in post-production in Culver City. The score was completed and delivered to MGM on April 1, 1956, and the film was released for a studio sneak preview soon afterward. The musician's union, however, objected to the soundtrack, and blocked the Barrons from being credited as "composers", hence the term "electronic tonalities".
This movie was filmed on the same sound-stage on which the movie The Wizard of Oz (1939) had been filmed seventeen years earlier; the set of Altaira's garden is a reuse of the Munchkin Village set from The Wizard of Oz.
MGM insisted on changes to Cyril Hume's script by adding comic relief scenes with the ship's cook (played in the final film by Earl Holliman). Among these scenes was one in which Robby the Robot responds to the cook's complaint about the lack of female companionship by bringing him a female chimp. The scene was reportedly not filmed.
This film marked one of the first times a science fiction project had received a large budget. The genre had rarely been taken seriously by studio executives, and had generally received the most meager of budgets. The critical success of this film convinced many in the film industry that well-funded science fiction projects could be successful. Film historian Ben Mankiewicz has claimed that Forbidden Planet's success made future big-budget science fiction films possible.
The model of the "flying saucer" style Earth space cruiser was retained by the MGM prop department and subsequently used in a number of productions on the MGM lot, including Twilight Zone: To Serve Man (1962). Robby the Robot, his ground transporter, and crew uniforms would be used on Twilight Zone (1959) as well.
The film was originally conceived and approved by MGM's Dore Schary, himself no fan of science fiction, as a B-picture. The studio's art department, still headed by veteran Cedric Gibbons pulled out all stops. The budget ballooned to $1.9 million and barely managed to break even amid a dismal year for the studio. The relative failure of the film was cited as a reason for Schary's ouster soon after.
The opening narration states, "In the final decade of the 21st century, men and women in rocket ships landed on the Moon." That would have been about 140 years after the film's 1956 date. The actual first Moon landing occurred only 13 years after the film.
Robert Kinoshita, who is credited as building Robby the Robot, was also Art Director for the TV series Lost in Space (1965). Many of the Lost in Space Robot's features are similar to Robby's: glass "head" with animated elements; rotating antenna "ears" (although LiS Robot's ears rarely moved after the pilot episode); flashing light "mouth"; chest panel with more animated elements. For that matter, much of the layout of Forbidden Planet's spaceship is mirrored by LiS's Jupiter 2: saucer shape; integral landing gear/entry stairs; lower external dome with animated lights; central, plexi-domed navigation station; vertical hibernacula arranged along perimeter. In addition, Robby and the LiS Robot had a couple of "family reunions" in two LiS episodes: Lost in Space: War of the Robots (1966) and Lost in Space: Condemned of Space (1967).
David Rose, composer of light orchestral music such as "Holiday For Strings", was originally hired to write the score. He was relieved of his contract by producer Dore Schary in December 1955 when Schary discovered avant-garde electronic music creators Louis Barron and Bebe Barron in a nightclub in Greenwich Village, New York, and hired them on the spot. The only confirmed piece of music which still remains from Rose's discarded original score is his Main Title Theme, which he released as a single on MGM Records in 1956.
The scene in which the image of Altaira appears in the Krell's "plastic educator" device was achieved with several special effects, including superimposed film footage of the charge from an electrical generator, hand-drawn animation, and a traveling matte cut from film footage of Anne Francis.
Early in the film just after the space ship drops out of light speed, the ship's speed is announced as .38 of light speed. That is still 254 million MPH. At that speed, the ship could fly the path of a single Earth orbit in 2 hours, 17 minutes. Or travel from Los Angeles to New York in 4/100ths of a second.
Writers and special effects artists Irving Block and Allen Adler originally conceived of this film as a B picture and brought it to Allied Artists, which turned it down. They then decided to try their luck at MGM, then still Hollywood's most prestigious studio, which had not produced a science fiction film since The Mysterious Island (1929). To their surprise, studio chief Dore Schary green-lighted the project, immediately catapulting the film to the status of a major production.
The planet on which Edward and Altaira Morbius live is Altair IV, which according to Star Trek: Deep Space Nine (1993) is also a Federation planet. Director Fred M. Wilcox consulted with scientists before making the decision that the planet's sky would be green.
The reaction from the preview audience to this film was so positive that it was released as it was, with no further changes to the movie. The result is that there are several rapid takes toward the story's end.
Studio chief Dore Schary and producer Nicholas Nayfack were unsure about releasing the film with a solely electronic score by Louis Barron and Bebe Barron. A rough cut of the film was previewed with the electronic score. The audience reaction to the film overall was so favorable that Dore Schary ordered the rough cut to be released with the electronic score and no further editing.
In addition to animating the monster that invades the camp, Walt Disney studios artist Joshua Meador provided approximately 29 other animation effects depicting laser beams and other forms of visual energy.
Apart from the electronic tonalities composed by Louis Barron and Bebe Barron, the music score known to many as "Forbidden Planet Fanfare - Parts 1 & 2" on the original 1956 theatrical trailer was composed by André Previn, and pieced together seamlessly by an MGM music editor. The music was originally written by Previn for the MGM films Scene of the Crime (1949) and Bad Day at Black Rock (1955).
To increase the sense of depth, the opening image of the spaceship approaching the camera is actually composed of two shots: the first of a small model, the second of a larger model travelling on the same track. The ship passes into and out of a shadow to conceal the cut.
MGM had had a full animation department at one time but by 1956 it was largely dismantled. Critical animation effects (landing beam, weapons, Robby overloading, the Id Monster) were provided by Joshua Meador on loan to MGM from Walt Disney Pictures. Meador's recognizable style can be readily discerned from that of the other three effects animators working on Alice in Wonderland (1951) and in other Disney releases.
The special effects artists used split-screen traveling mattes to make images appear and disappear, such as the piece of fruit Morbius lobs at the "household disintegrator beam" and the tiger that Commander Adams vaporizes.