Source can be found at the end of the article. [Facts from IMDB]
1/21. The Spice Girls were originally considered for the roles of the Muses.
2/21. When James Woods auditioned for his role as Hades, he shocked the writers so much with his talent that the writers changed him from a very menacing, slow talking villain to an easy-to-anger, fast talking Hollywood agent/used car salesman sort of villain.
3/21. When Phil and Hercules first enter Phil's hut, Hercules bangs his head on what Phil tells him is the mast of the Argo. In Greek mythology, Jason, the captain of the Argo, was killed when the mast hit his head. In the myths, Hercules was one of the heroes/sailors on the Argo.
4/21. While the movie is about Greek myths, Hercules is actually the protagonist's Roman name. Heracles is his Greek name.
5/21. The role of Hades is one of James Woods' all-time favorite characters. So much so, any time Disney needs him to reprise the character for any cartoon show and video game, such as Hercules: The Series or Kingdom Hearts, he has agreed to do so.
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6/21. In the scene where two kids were trapped under a boulder, they said "someone call IXII". Though pronounced as letters, this is a reference to the Roman numerals IX, I, I, or 911.
7/21. Pegasus was created by the blood of Medusa, not clouds (as seen in the film) on Olympus.
8/21. The "Air Herc" sandals brand is a reference to the Nike Air footwear line. Moreover, the Hercules mosaic showing Hercules running resembles an actual Nike print ad. Nike was the Greek goddess who personified triumph and inspired the shoe company's name.
9/21. When Hercules and Phil first meet, Phil says "Two words: I am retired," much to the confusion of Hercules. However, in Greek, "I am retired" is actually only two words.
10/21. Hercules animators wore out 72,000 pencils and used 1 million sheets of animation paper.
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11/21. Phil claims that he trained Perseus, Theseus. Odysseus, and Achilles. He then says that his dream was to train a hero so great that the gods would hang a picture of him in the stars. Perseus, the Greek hero famed for killing the Gorgon Medusa, does actually have a constellation.
12/21. James Woods (Hades) ate watermelon to make the slurping sounds more authentic in the scene where Hades is eating worms.
13/21. In Greek mythology, there were nine Muses. The five shown here are: Calliope (epic poetry), Clio (history), Melpomene (tragedy), Terpsichore (dance), and Thalia (comedy).
14/21. During the song "Zero to Hero" the muses talk about Hercules doing things like endorsing products and making personal appearances for money. It may seem anachronistic to the point of being silly on purpose, but even back in ancient Greece and Rome, famous athletes pretty much did all those things and were paid very well for them.
15/21. Animator Andreas Deja, who is best known for animating villains like Jafar in Aladdin (1992) and Scar in The Lion King (1994), requested the task of animating Adult Hercules, the hero.
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16/21. In the movie, Hades releases the Titans from a place at the bottom of the ocean. but in the original myth they were banished to Tartarus, which is in the deepest and worst section of the underworld.
17/21. Pain calls the potion that turns Hercules mortal 'Grecian Formula'. Grecian Formula is an actual product used to color greying hair.
18/21. Phil's appearance was partly inspired by Danny DeVito.
19/21. The first animated Disney film since Oliver & Company (1988) where the villain does not sing.
20/21. In the version for Latin America, the voice of Hercules is played by Ricky Martin and Megara is voiced by singer Tatiana.
21/21. The animators spent 6 to 14 hours to render a frame of the Hydra, depending on how many heads it had.
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