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.
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.
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.
Godzilla's appearance greatly changed for his 1962 appearance in the first color movie: King Kong vs. Godzilla. King Kong vs. Godzilla 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 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. The center dorsal fins were englarged 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.
Promotional artwork pasted images of the classic 1933 Kong over images of the original Godzilla destroying Tokyo. Apparently the publicity department thought the Kong suit used in the film looked too ridiculous to be used for advertising the movie!
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.
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.
The trivia items below may give away important plot points.
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.
Although fans of both Kong and Godzilla argue to this day, Toho has declared that King Kong was meant to win. Kong was much more popular than Godzilla at this time, and was the obvious choice to win audiences over.
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.