Enforced Method Acting: When Haruo Nakajima (Godzilla's suit actor) slammed into the Castle during Godzilla's attack in Nagoya, some teeth from the Godzilla suit fell out. Also unknowingly at the time was that the impact of slamming into it also dislocated the jaw on the Godzilla suit, giving it the wobbly muzzle that is seen throughout the movie. SFX director, Eiji Tsuburaya liked the outcome of the shot so much that he keep it in the film. Also see Throw It In! below.
Playing Against Type: Kenji Sahara, who usually plays the young hero, is given the role of the villainous Torahata, a card-carrying financial swindler.
Throw It In!: Godzilla's head catching aflame after having napalm dropped on him was unintentional, but the take was good enough to be kept in.
What Could Have Been: The movie itself was drastically different than what we got with the movie now. Godzilla was intended to have a bigger role. He was intended to have washed ashore and be a tourist attraction instead of Mothra's Egg, and upon awakening, was to attack the Country of Rolisica, instead of Nagoya. There were no Mothra Larva at all in the movie, only the Imago Mothra, who would've fought with Godzilla at the film's climax and won.