- Awesome, Dear Boy: Harry Hamlin had to choose between doing this film and an adaptation of Tristan and Isolde. He chose this because Laurence Olivier was in it. Tristan & Isolde was not made until 2006.
- Colbert Bump: It's been said that most people born from the 80s onwards owe their knowledge of Medusa to this film. She rarely appears in media made before this. Notably the Hammer Horror film The Gorgon doesn't use her.
- Creator Backlash: Ray Harryhausen viewed the stop motion on this film as some of his weakest work.
- Dawson Casting: Twenty-eight year old Harry Hamlin as twenty year old Perseus.
- Executive Meddling: The original script had more Family-Unfriendly Violence and more nudity on the part of Andromeda (she was supposed to remain nude throughout the entire Kraken sacrifice scene). These elements were toned down in pre-production when the British censors reviewed the script and told the studio that they would give the film an "AA" (no one under 14 admitted) rating if they put those scenes in the film.
- Follow the Leader: Bubo was used in the film to capitalize on the popularity of R2-D2. He does feature in Greek mythology though.
- Money, Dear Boy: Part of the reason Laurence Olivier was cast as Zeus. Probably the same with Maggie Smith, who is on record that she prefers stage work but does films because they pay better.
- Playing Against Type: Maggie Smith as a vengeful goddess who isn't particularly funny — and is actually rather terrifying.
- Real-Life Relative: Maggie Smith was married to the film's screenwriter.
- Romance on the Set: Harry Hamlin and Ursula Andress began a relationship during the production of this movie, which produced a son, Dimitri Hamlin, born in 1980 after completion of principal photography.
- The Shelf of Movie Languishment: The film was completed in 1979, but not released until 1981.
- Stillborn Franchise: According to Ray Harryhausen's official website, a sequel to this movie, titled Force of the Trojans, was pitched to MGM in 1984, but further development of the picture never eventuated.
- Wag the Director: The original script called for Perseus to cut off Medusa's head simply by throwing his shield at her, in an attempt to appease UK Standards and Practices censors (as the producers felt that the hero decapitating someone would not be appropriate for children in the audience). Harry Hamlin was apparently resistant to the idea from the beginning, as it wasn't in keeping with the actual Greek Mythology. When the day came to film the scene and it still hadn't been changed, he threatened to quit the film and fly home. He remained in his trailer, much to the producer, director, and Ray Harryhausen's annoyance. In the process of trying to coax him out, he was gradually able to get some of the other crew members on his side, which resulted in the scene being rewritten accordingly.
- What Could Have Been:
- The original storyline focused just on Perseus and Andromeda's romance, linking together a number of myths. Once Ray Harryhaussen was brought on board, the story was changed to feature more monsters.
- Richard Chamberlain, Malcolm McDowell and Michael York were considered for Perseus. Arnold Schwarzenegger was briefly considered, but producer Charles H. Schneer felt that, with the exception of Hercules, Greek heroes were athletic, but not overly muscular, and relied more on cunning than strength. He also felt that casting a very muscular actor was a cliché, hearkening back to the cheesy Italian sword and sandal films made in the 1950s and 1960s.
- Rex Harrison was asked to play Poseidon but rejected the role as being too tiny.
- John Gielgud was considered for Ammon.
- Initially, Calibos had no dialogue and was a purely stop-motion character. After a rewrite to the script, dialogue was added and the role was given to Neil McCarthy.
Trivia / Clash of the Titans (1981)