Filming
Special effects director Eiji Tsuburaya was planning on working on other projects at this point in time such as a new version of a fairy tale film script called Princess Kaguya, but he postponed those to work on this project with Toho instead since he was such a huge fan of King Kong. He stated in an early 1960s interview with the Mainichi Newspaper, "But my movie company has produced a very interesting script that combined King Kong and Godzilla, so I couldn't help working on this instead of my other fantasy films. The script is special to me; it makes me emotional because it was King Kong that got me interested in the world of special photographic techniques when I saw it in 1933."
Early drafts of the script were sent back with notes from the studio asking that the monster antics be made as "funny as possible". This comical approach was embraced by Tsuburaya, who wanted to appeal to children's sensibilities and broaden the genre's audience. Much of the monster battle was filmed to contain a great deal of humor but the approach was not favoured by most of the effects crew, who "couldn't believe" some of the things Tsuburaya asked them to do, such as Kong and Godzilla volleying a giant boulder back and forth. With the exception of the next film, Mothra vs. Godzilla, this film began the trend to portray Godzilla and the monsters with more and more anthropomorphism as the series progressed, to appeal more to younger children. Ishirō Honda was not a fan of the dumbing down of the monsters. Years later, Honda stated in an interview. "I don't think a monster should ever be a comical character," and "The public is more entertained when the great King Kong strikes fear into the hearts of the little characters." Honda also believed that if Godzilla acted like a human, it showed off the fact that it was a man in a suit. The decision was also taken to shoot the film in a (2.35:1) scope ratio (TohoScope) and to film in color (Eastman Color), marking both monsters' first widescreen and color portrayals.
Toho originally planned to shoot the film on location in Sri Lanka, but abandoned the idea after being forced to pay RKO roughly ¥80 million ($220,000) for the rights to the King Kong character, which forced the company to scale down their original production costs. The bulk of the film was shot on the Japanese island of Izu Ōshima instead. The movie's production budget came out to ¥150 million ($420,000).
Suit actors Shoichi Hirose (as King Kong) and Haruo Nakajima (as Godzilla) were given mostly free rein by Tsuburaya to choreograph their own moves. The men would rehearse for hours and would base their moves on those of professional wrestling (a sport that was growing in popularity in Japan), in particular the moves of Toyonobori.
During pre-production, Tsuburaya toyed with the idea of using Willis O'Brien's stop-motion technique instead of the suitmation process used in the first two Godzilla films, but budgetary concerns prevented him from using the process, and the more cost-efficient suitmation was used instead. However, some brief stop-motion was used in a couple of quick sequences. Two of these sequences were animated by Minoru Nakano.
A brand new Godzilla suit was designed for this film and some slight alterations were done to its overall appearance. These alterations included the removal of its tiny ears, three toes on each foot rather than four, enlarged central dorsal fins, and a bulkier body. These new features gave Godzilla a more reptilian/dinosaurian appearance. Outside of the suit, a meter-high model and a small puppet were also built. Another puppet (from the waist up) was also designed that had a nozzle in the mouth to spray out liquid mist simulating Godzilla's atomic breath. However the shots in the film where this prop was employed (far away shots of Godzilla breathing its atomic breath during its attack on the Arctic Military base) were ultimately cut from the film. These cut scenes can be seen in the Japanese theatrical trailer. Finally, a separate prop of Godzilla's tail was also built for close-up practical shots when its tail would be used (such as the scene where Godzilla trips Kong with its tail). The tail prop would be swung offscreen by a stagehand.
Sadamasa Arikawa (who worked with Tsuburaya) said that the sculptors had a hard time coming up with a King Kong suit that appeased Tsuburaya. The first suit was rejected for being too fat with long legs giving Kong what the crew considered an almost cute look. A few other designs were done before Tsuburaya would approve the final look that was ultimately used in the film. The suit's body design was a team effort by brothers Koei Yagi and Kanji Yagi and was covered with expensive yak hair, which Eizō Kaimai hand-dyed brown.
Because RKO instructed that the face must be different from the original's design, sculptor Teizō Toshimitsu based Kong's face on the Japanese macaque rather than a gorilla, and designed two separate masks. As well, two separate pairs of arms were also created. One pair were extended arms operated by poles inside the suit to give Kong a gorilla-like illusion, while the other pair were at normal arms-length and featured gloves that were used for scenes that required Kong to grab items and wrestle with Godzilla. Suit actor Hirose was sewn into the suit in order to hide the zipper. This would force him to be trapped inside the suit for large amounts of time and would cause him much physical discomfort. In the scene where Kong drinks the berry juice and falls asleep, he was trapped in the suit for three hours. Besides the suit with the two separate arm attachments, a meter-high model and a puppet of Kong (used for closeups) were also built. As well, a huge prop of Kong's hand was built for the scene where he grabs Mie Hama (Fumiko) and carries her off.
For the attack of the giant octopus, four live octopuses were used. They were forced to move among the miniature huts by having hot air blown onto them. After the filming of that scene was finished, three of the four octopuses were released. The fourth became special effects director Tsuburaya's dinner. These sequences were filmed on a miniature set outdoors on the Miura Coast, According to assistant special effects cinematographer Kōichi Kawakita, the crew had difficulty getting the live octopuses to move. Along with the live animals, two rubber octopus puppets were built, with the larger one being covered with plastic wrap to simulate mucous. Some stop-motion tentacles were also created for the scene where the octopus grabs a native and tosses him. These sequences were shot indoors at Toho's studios.
The special effects crew filmed a sequence in which Godzilla rampages through Takasaki, Gunma Prefecture, and destroys the Takasaki Kannon Statue, but this scene was cut from the final film. Despite this scene being cut, Godzilla is still said to have passed through Takasaki in the film.
Since King Kong was seen as the bigger draw and Godzilla was still considered a 'villain' at this point in the series, the decision was made to not only give King Kong top billing but also to present him as the winner of the climactic fight. While the ending of the film does look somewhat ambiguous, Toho confirmed that King Kong was indeed the winner in their 1962–63 English-language film program Toho Films Vol. 8, which states in the film's plot synopsis, "A spectacular duel is arranged on the summit of Mt. Fuji and King Kong is victorious. But after he has won..." While producer Tomoyuki Tanaka stated in a pair of books he wrote in the 1980s that he believed the battle ended in a draw, Toho still maintains that Kong was the victor on their global website in 2023.
