It's not generally known among non-Godzilla enthusiasts, but "Mothra" had her own movie in 1961. Mosura (1961) was one of the major monster films Toho made during Godzilla's rest from the silver screen from 1955-1962. Other Toho monsters that originally had their own separate movie would take the same path as Mothra. Rodan, Manda, Baragon, and Varan would appear in future Godzilla movies, but Rodan though would be the only monster out of these four that would take on any major roles.
The famous scene where Godzilla destroys the castle in Nagoya actually had to be filmed several times, and the castle had to be built twice. To make the castle fall correctly, two men are sitting behind pushing it with long sticks. The first time the scene was shot, the two men pushed too soon, and the castle collapsed before Godzilla even touched it. The miniature castle then had to be rebuilt. The second time the opposite problem happened, and the two men missed their cue (you can see when Godzilla slams into the castle that it doesn't move). A second take had to be done with Godzilla clawing away at it as the two men push it over from behind. The moment where Godzilla's foot slides on the ledge (causing him to fall into the castle) has been thought by many to be an accident. According to Haruo Nakajima (the actor inside Godzilla) the fall was actually scripted that way and done on purpose.
The year 1964 is the only when Toho released two Godzilla movies in one year. Right after this film, Toho began working on San daikaijû: Chikyû saidai no kessen (1964), which debuted that December.
This is the final Showa Era film in the series where Godzilla is a malevolent figure. In later films he would be portrayed as a protector of Earth and the only time he would cause destruction was when he was controlled by aliens. However, when the series was rebooted for the Heisei Era, starting with Gojira (1984), Godzilla was returned to his original portrayal as a destructive menace.
When the sequence of Godzilla being firebombed was being shot, during one of the takes the flaming liquid splashed up onto Godzilla's head. The fuel quickly burned away and the flames went out. Inside the suit, performer Haruo Nakajima was unaware of the accident, since the head of the suit is well above Nakajima's own head. Godzilla's head aflame became an unplanned bonus to the effects shot and it was included in the final cut.
When Godzilla is wading in the water heading for Iwo Island, he is only shot from behind. This is due to the fact that Haruo Nakajima was actually wearing the suit from Kingu Kongu tai Gojira (1962) instead of the Mosi Goji suit. It had already been planned to use the suit from this film for the next film in the series, San daikaijû: Chikyû saidai no kessen (1964) and in order to save wear and tear it was decided to use the older suit.
The Torada Beach sequence, in which Godzilla is attacked by the U.S. Navy, was shot by Toho for the U.S. release. It is one of the few examples where footage of Godzilla was shot outdoors, away from the controlled environment of the studio.
The adult Mothra is this film is the same creature that was featured in Mosura (1961).
The Mosugoji suit is a favorite among fans. After the more comedic approach to Godzilla in Kingu Kongu tai Gojira (1962), Toho decided to return its big star back to his original, evil, marauding depiction for this film. The Mosugoji suit, designed by Teizô Toshimitsu, was very different in appearance than its predecessor, Kingoji. Godzilla had less reptilian features than the Kingoji suit. The Mosugoji suit was also less bulky. The snout was slightly shorter, and the yellow around the pupils was changed to white. The tail was also made longer, and the skin featured a slight greenish tinge.
First Godzilla film without newly-shot American footage added for the American release. The scene in which Godzilla is attacked by the American military only exists in the American cut, but this footage was shot by Toho during the original Japanese shoot.
With this film, the name of Mothra's island changes from "Beiru" to "Infant Island".
In one of the early drafts of the script, it was Godzilla who washed ashore and not Mothra's egg. Torahata, thinking that Godzilla was dead, would have put Godzilla on display and eventually he would revive and start another rampage. However, that idea was abandoned due to the fact that it wouldn't have made sense for anyone to come so close to a highly radioactive creature like Godzilla.
This was the first film in the series to be dubbed for American release by Titra Sound, the same studio that was also responsible for dubbing several Japanese animation features as well as "Speed Racer".
In the U.S. release of the film Mothra's official name is never used. Instead she is simply called "The Thing" in both the moies title and all dubbed english dialogue with the exception of the song sung by the twin fairies to summon and communicate with the monster.
Because of the commercial success of Mosura tai Gojira (1964), Toho decided to release another Godzilla film in 1964, San daikaijû: Chikyû saidai no kessen (1964). The Mosugoji suit, though, needed to go through some repairs. During the shooting of this film, the mouth had become wobbly and would not work properly. The head had also caught fire, which didn't help the suit much. So the head of the Mosugoji suit was replaced by one of a sturdier design. "Mosugoji II," as some fans refer to the repaired suit, also had more mobility than the original Mosugoji.
In the scene towards the end, where Godzilla's head pops up behind the mountain (behind the policeman), the Godzilla "theme" music begins to play. Composer Akira Ifukube felt that music wasn't needed in that scene because, as he said, "Godzilla's appearance is already impressive enough." However, director Ishirô Honda decided to include the music cue anyway. Upon the first screening, when the scene appears with the music, it is said that Ifukube gave Honda a very "disapproving" look. However, it was the only moment in their professional careers together that they ever had a disagreement.