Emma Thompson almost made an accidental uncredited 'cameo' in this movie while visiting friend Liam Neeson on the set. Thompson, who'd been filming Nanny McPhee Returns (2010) in an adjacent studio, went to visit Neeson during a break just as Neeson was about to shoot a scene with Ralph Fiennes and Danny Huston. Unable to exit the set fast enough as the cameras began to roll, Thompson, in clumsy Nanny costume, had to hide behind Huston's throne during the take so she would not be picked up by the cameras.
The mechanical owl Bubo from Clash of the Titans (1981) has a cameo as the toy Perseus picks up before he leaves on his quest. According to the filmmakers, the cameo was widely debated as to whether to keep it in the film or not. It was eventually decided to keep it in the film to please the fans of the original film.
The word Kraken (sea monster) is Norwegian/Swedish, not Greek. Early script drafts considered changing it to its Hebrew counterpart Leviathan (famous from the Bible hymn of Job 41). It was changed back to Kraken as a tribute to the original Clash of the Titans (1981) tagline "Release the Kraken!" Surprisingly, the creature's Greek name, Cetus, was never considered.
Although Greek mythology contains different versions of the story of Perseus, the film deviates from all of them in some parts. Perseus supposedly flew to an island using winged sandals he borrowed from Hermes, and Medusa was only one of three Gorgons (i.e. half-woman half snake creatures with serpents for hair, the other two were called Stheno and Euryale).
In later interviews, Louis Leterrier would disown the film, claiming that the 2D-to-3D conversion was a studio decision that was forced on him despite his attempts to point out that it wasn't working. He now says "it's not my movie" and the experience of dealing with that made him choose not to return for the sequel.
Concept designer Aaron Sims considers designing Medusa his most difficult task: "Are they all the same snake in her hair? Do they look more like hair? Are they different in silhouette or in light? And how much of a human face does she have, or is it more like a snake? I worked on one design, and people said it reminded them of Lord Voldemort because there was no nose. You have to be careful so it still looks like it's an original idea."
A replica of the owl Bubo used in Clash of the Titans (1981) was used for this film. Sam Worthington hated it and threatened to destroy it when director Louis Leterrier wasn't looking. According to Leterrier, "[Worthington] would say: 'This is ridiculous! This is a ridiculous thing to have in the movie! You're going to ruin my career with that owl!'"
This film begins with a set of constellations portraying history's events. Clash of the Titans (1981) ended with a set of constellations portraying history's events (though not the same constellations).
Sam Worthington stated in an interview that Perseus was meant to start out the film with long hair but would cut it short before going off on the journey. This idea was scrapped because they felt the scene didn't work and Perseus was given short hair throughout the whole film.
Despite the mythological nature of this film, it draws heavily on actual Minoan (Crete, approx. 2700-1450 BC) archeology for its props and costuming. The characters' long, curled hair is seen in palace frescoes from Knossos and other sites. The tiered dresses of the dancing girls in Argos' palace are PG-13 versions of the Snake Goddess figurines' costumes from the same palace site. With Andromeda's final costume, a nearly perfect replica of the Bee Pendant from Malia graces her neck; the only difference is that the movie version is constructed from silver rather than the original gold.
While the film is primarily based on Greco-Roman mythology, aspects of it are drawn from other cultures. The Kraken comes from Norse mythology, and the Djinn originated in the Arabian/Oriental regions, while Cepheus's snipe about Andromeda being a missionary might hint at the coming Christianity (Christians often served as missionaries).
Originally Perseus was envisioned as being more of a young slender man. Louis Leterrier had watched Sam Worthington in the film Somersault (2004) and thought that he would be ideal and was therefore somewhat taken aback when he met Worthington. The actor had bulked up considerably for his roles in Avatar (2009) and Terminator Salvation (2009).
Louis Leterrier originally wanted to make the film in 3D but Warner Brothers nixed the idea as it was too expensive. After the success of Avatar (2009), the studio reconsidered. At this stage, however, most of the filming had been done so the 3D conversion was a retrofit.
The thirteenth film released in select D-BOX enabled cinemas, located in the US and Canada. In D-BOX's words, the motion control technology "adds to the movie's plot and underlying themes of fear, terror and explosive action by offering realistic sensations during most of the film's action scenes."
Early in the movie, Perseus told Draco that he would not use anything from the gods but later in the movie when he mets Zesus for the first time, Perseus accepts the gold coin from Zeus that he used as a bribe for the ferryman.
Mads Mikkelston, Gemma Arteton and Ralph Fiennes have all starred in Daniel Craig Bond films, Mikkelston as Le Chiffe in Casino Royale (2006), Areton as Fields in Quantum of Solace (2008) and Fiennes as Mallory in Skyfall (2012) and M in Spectre (2015).
The trivia items below may give away important plot points.
According to the director, the movie was meant to end with Perseus and Andromeda ending up together (as in all previous tellings of the Perseus story) with Perseus and Io's relationship being purely platonic. However the studio disliked this idea and the movie was re-shot to have Perseus and Io end up together.
When Acrisius is disfigured, he takes the new name of Calibos. Calibos was a character who first appeared in Clash of the Titans (1981). He is named for Caliban, the antagonist of William Shakespeare's play The Tempest (1611).
In Greek mythology, Perseus and Andromeda had seven sons, one of whom is Perses, who became an ancestor of the emperors of Persia, and from whose name "Persia" is supposedly derived. Io (who in Greek mythology, is Perseus' great great great great great great great grandmother) is portrayed by Gemma Arterton, who appeared in Prince of Persia: The Sands of Time (2010), as the love interest of a Persian prince.