There were four live octopuses used in the fight sequence with Kong and the natives, as well as a plastic model. Hot air was blown on them to get them to move and after the filming of the scene was finished, most of them were released except for one, which Eiji Tsuburaya had for dinner.
This film introduced Godzilla's more well-known and standardized high-pitched roar (which was actually a mix of two of his original 1954 roars, sped up by several cycles). This was a way to tone down Godzilla's darkness from the previous two movies. This ultimately became Godzilla's roar for the remainder of the Showa Godzilla film series, and was also used in the VS/Heisei and Millennium Godzilla film series, albeit with tweaks.
This film marks a number of firsts for King Kong and Godzilla films: this film was the first time either King Kong and Godzilla were filmed in color and the first time either filmed in wide-screen. This film was also the third film for both King Kong and Godzilla (although this film isn't considered a sequel to the original King Kong or Son of Kong).
Originally, Honda had thought about using stop motion animation instead of men in costumes, but due to budgetary concerns those plans were scrapped. However, there are a couple of scenes where the technique is used; first in the fight with the giant octopus as it grabs one of the natives and second in the second battle between Kong and Godzilla when Godzilla gives Kong a dropkick.
The idea for this movie was spawned by Willis O'Brien, who had done the special effects for King Kong (1933). In the late 1950s, he tried to drum up interest in a sequel to be titled King Kong vs. Frankenstein. The Frankenstein would have been a giant monster created from different animals. Unable to find an American studio interested in the project, producer John Beck offered the idea to Toho who replaced Frankenstein with Godzilla.
As of August 2011 this is only remaining Showa era film where the original Japanese version has not been released in the United States. All the other films in the series have been released in both their original Japanese and English versions.
Special effects director Eiji Tsuburaya deliberately gave King Kong a semi-comical personality, because he did not want Kong to frighten young children, and wanted the general audience to root for Kong over the more frightening Godzilla.
Godzilla's appearance greatly changed for his 1962 appearance in the first color movie: King Kong vs. Godzilla (1962). King Kong vs. Godzilla (1962) was made more as a comedy film than having the "sense of terror" theme in the two previous movies. Because of that, Toho decided to make Godzilla less demeaning. While some American posters (and some Japanese merchandise) of the previous two black and white Godzilla movies showed Godzilla as green, the Kingoji suit revealed Godzilla's true color: charcoal gray. The previous two Godzilla suits were painted brown. On this suit, Godzilla's ears were taken away, and instead of having four toes on each foot, Godzilla had three; a more simplified characteristic for Godzilla, for the remainder of the Showa Godzilla film series. The center dorsal fins were enlarged and the two side dorsal fins decreased in size. The body of Godzilla was bulkier than the last two suits. The head was made longer and a slight frown was added to the side the mouth, a feature that would be seen in some later suits. The pupils were enlarged and the eyes sported a yellow-reddish color. The new features on Godzilla gave him an alligator-like appearance.
Early Toho promotional photos (while production started) used pictures of the original 1933 King Kong and the 1954 (and 1955) Godzilla pasted onto background scenery. These were also seen on posters overseas, including in the US.
The first Godzilla film to use Toho's "Big Pool" (originally built for the studio under the supervision of special effects wizard Eiji Tsuburaya for Hawai Middowei daikaikûsen: Taiheiyô no arashi (1960)) for water scenes. The Big Pool would be used for all water scenes in the Toho-produced Godzilla films (and other Toho tokusatsu films and TV series), until Gojira: Fainaru uôzu (2004), upon the completion of which the Big Pool was demolished.
In the Japanese version, the Seahawk submarine story is being followed by the "Wonderful World Series", a science show sponsored by Pacific Pharmacuticals instead of the United Nations news show in the American version. The US version also overdubbed the English-speaking actors from the original film.
This marked the series debuts for both Jun Tazaki and Kenji Sahara. Tazaki would appear in five Godzilla films while Sahara would appear in twelve including eight in the Showa era, three in the Hesei era and one in the Millenium era. Also, even though Sahara did appear in the first film, his appearance only amounted to a cameo of only a few seconds.
In the American version by Universal International, footage from The Mysterians (1957) was used. The Mysterians' satellite was passed off as the "satellite" for the UN World News program. Scenes with people evacuating, with military helping them onto vehicles, is placed in the middle of Kong's initial rampage in Chiba (or Tokyo in the US version). A shot of Mount Fuji from that film was used in the beginning of the scene when the human characters are driving on their way to see Kong fight Godzilla in the climax. And when Kong and Godzilla fall into the ocean near the end, various shots of the resulting tidal wave, earthquake and landslide were used from that film.
This became one of the most popular films internationally in the original Godzilla series. In the early 1990s, Toho and producer Tomoyuki Tanaka planned to remake this film. However, the new owner of the RKO library, and the underlying copyrights, Turner Entertainment, wanted a license fee for the use of King Kong that was too high for Toho. Toho changed the concept and produced "Godzilla vs. King Ghidorah" (1991) instead.
Godzilla has three toes in this movie instead of four. In Japan, the number four is an unlucky number, so for his more positive or comical appearances in the first series of Godzilla films, he would keep having only three toes. When Toho later brought Godzilla back to his darker, more serious roots in the 80s, he went back to having four toes again.
The trivia items below may give away important plot points.
Although fans of both Kong and Godzilla argue to this day, Toho has declared that King Kong was meant to win. Apart from the fact that Godzilla was still a villain, Kong was much more popular than Godzilla at this time, and was the obvious choice to win audiences over.
In the final fight scene, when King Kong throws Godzilla over his shoulder, Shoichi Hirose didn't throw an empty suit, but actually threw it with Haruo Nakajima still inside. It was Hirose's way of proving his was the stronger of the two.
During the final fight between King Kong and Godzilla, King Kong tries to shove a tree down Godzilla's throat. This is a tribute to the fight between Kong and the Tyrannosaurs Rex from the original King Kong (1933), where a famous publicity still from that encounter shows Kong shoving a tree into the T-Rex's mouth.