Gregory Peck auditioned for the role of Captain Nemo.

Charles Boyer was originally considered for the role of Professor Arronax. He had to turn down the role due to prior commitments.

Peter Lorre claimed that the giant squid got the role that was usually reserved for him.

Walt Disney briefly considered English actor Ralph Richardson for the role of Captain Nemo.

Walt Disney originally considered making this film as an animated feature; the detailed pre-production sketches by artist Harper Goff, as well as Goff's enthusiastic suggestion that it be done as live action feature, convinced him otherwise.

Richard Fleischer was surprised at being considered for the director's chair for this film, as he was the son of Disney's biggest competitor, Max Fleischer. He approached Walt Disney to inquire if Disney knew who he was. Disney told him that he was well aware of who he was and hired him because he thought he was the best man for the job. Richard Fleischer also asked his father if he minded having his son working for his rival but Max Fleischer made no objection and even asked Richard to tell Disney that he thought he had made an excellent choice for his director!

According to Kirk Douglas in his autobiography, "Ragman's Son", the scene at the beginning of the film where Ned Land strolls up (with a beautiful girl on each arm) to the lecturing sailor - and the ensuing fight - was written into the film at his request. At the time this film was shot, Douglas was in the prime of his career and very concerned about promoting and preserving his reputation with the movie-going public as both a dashing ladies man and a "macho" actor known for tough, physical roles that showcased his image as an action hero (fight scenes and so forth). When he first read the first draft script for this film, he was somewhat disappointed to find that his character Ned Land made no appearances with any women whatsoever, had no real rousing fight scenes, other than the battle with the squid, and spent most of the movie talking instead of being in action. Having expressed these concerns both to Walt Disney and director Richard Fleischer, the San Francisco street scene at the beginning of the film - featuring his character involved with two beautiful women in addition a rousing action/fight scene - was added especially for him (in at least two later films Douglas also made sure his character would be seen with two beautiful women at the same time and be inv

Actors portraying the cannibals chasing Ned Land painted humorous messages on their foreheads (not legible on-screen). In particular, one actor wrote "Eat at Joe's" while another actor behind him wrote "I ate Joe".

Although Percy Helton was billed in the opening credits, his role (as a coach driver) was almost entirely eliminated; in the release version, he appears in only one shot (no dialogue), waiting for passenger Lukas to finish his interview with the reporters.

Although Nemo dies in this movie, his fate was left unknown in the original Jules Verne novel, and he reappeared in the sequel, Mysterious Island.

Average Shot Length (ASL) = 9 seconds

Cast members carried herring in their pockets to reward Esmerelda the seal after their scenes with her. Director Richard Fleischer was especially amused when the distinguished James Mason had to reach into his pocket after a shot. Fleischer said the cast smelled like the Fulton Fish Market.

CASTLE THUNDER: Right at the start of the storm sequence with the giant squid.

During his 'Whale of a Tale' tune, two of the girls Ned sings about are named 'Mermaid Minnie' and 'Typhoon Tessie'. Minnie and Tessie were the two girls on his arms at the beginning of the film - you can hear him shout their names at one point during the street brawl.

During the underwater shooting of the Treasure Chest scene, an unscripted nurse shark approached Ned, Conseil and the camera crew. All underwater personnel, as seen in Disney's Wonderful World of Color Sunday Night television episode highlighting the upcoming film, attempted to warn the shark off, but the interruption was considered so exciting that portions were left in the final film.

In 1969, Captain Nemo's pipe organ, which had been on display in Disneyland, was redressed and now resides in the ballroom of Disneyland's Haunted Mansion. A duplicate was constructed for the ballroom of Walt Disney World's Magic Kingdom Haunted Mansion, which opened in 1971, and over a decade later another duplicate was built for Tokyo Disneyland.

In addition to his many contributions as the production designer, Harper Goff also taught Kirk Douglas to play the guitar for the "Whale of a Tale" number.

In the opening credits, the title banner does not have a comma between the first and second zeroes ("20000 Leagues Under the Sea"), although a comma does appear on the poster and all related print advertisements.

In the scene where Kirk Douglas arrives at the Nautilus on a lifeboat that lies upside-down in the water, a diver under the boat holds it to keep it steady.

In this film, the power source for the Nautilus is clearly implied as nuclear power. This is intimated by, among other things, the nuclear explosion of Nemo's island at the end of the film. In the original Jules Verne novel, however, Nemo explains to the professor a system which most resembles a fuel-cell battery originally powered by coal burning. Disney's decision on this matter was clearly influenced by the discovery of nuclear energy (i.e. the atomic bomb) only 10 years before this film. Meanwhile, in the 21st century, it is predicted that cars will run on fuel cell batteries and the original power source will be coal-fired power plants, which is ironically a return to Verne's ideas from 150 years ago.