Henry Cavill did not do any weight training for his preparation for this film. His physique was achieved though a a tabata style training regime consisting of only body weight exercises. This training methodology was chosen because the goal was to not put on much mass but to develop a very lean, highly defined physique.
Director Tarsem Singh told Henry Cavill after their first meeting that if he got the role of Theseus, he would have to develop an "eight pack", not a "six pack". Cavill trained for six months and developed his abdominal muscles into a full "eight pack" which was then displayed throughout the movie in numerous shirtless scenes. He brought down his body fat percentage to 6%, which he subsequently maintained throughout the shoot.
When the project was set-up, the financing was still being negotiated even though filming was set to begin just 6 months down the line. Lead star Henry Cavill straight-away began his intense training regimen for the film even though the producers said that they did not have money for the trainers yet. Cavill elected to pay the trainers out of his own pocket until funding could be acquired and continued training. 6 months later as the filming date approached, the producers told him that the financing had fallen through and that he should stop training so Cavill stopped his physical prep then. Six months later the producers told him that they had now acquired alternate financing and that he should start training again and so Cavill set out to achieve the required physique for the character a second time only for the financing to fell through yet again! The producers again asked him to stop for the moment. After some time the producers finally acquired concrete financing and when they called Cavill the third time to tell him to start his training, they realized that he had not stopped since the last time and had maintained his physique and was already ready to start filming. They said that Cavill effectively ended up building his body three times over three separate 6 month periods for this role before filming finally began and they saw this as the best demonstration of his commitment to the role and the movie.
When he was editing the film, Tarsem Singh would set his action to a guide track from the TV mini-series The Pillars of the Earth (2010). He felt that that score seemed to blend in very well with his vision so, when it came to choosing a composer for Immortals, Trevor Morris was the natural choice.
Henry Cavill, who is naturally hairy, had to shave his torso for the film. But he said that shaving had its advantages as he was able to see all the muscle definition that he had achieved for this role.
The filmmakers state that they chose to change many aspects of the ancient Greek stories to reflect the fact that the Greeks themselves often revised their mythology to conform to new ideas and knowledge. One aspect of this film which is specifically addressed in the special features is the portrayal of Zeus and Poseidon as taking the form of young men rather than old men.
The trivia items below may give away important plot points.
The story is very loosely based on Greek mythology, combining elements of various myths. In the myths, Theseus was King of Athens (the capital of Greece) and (in some versions) a son of Poseidon (Greek god of sea and Zeus' brother), although his legal father was King Aegeus of Athens (namesake of the Aegean Sea). Theseus killed the Minotaur (a creature with the body of a man and a head of a bull imprisoned in a labyrinth, depicted here as a man with a helmet made of wires and horns) with the help of princess Ariadne whom he later married. Hyperion is from a different myth, and was the name of one of the 12 Titans. There were only 12 titans in the myths and they were immortal and unkillable. There was a war in the heavens and the Titans lost. Zeus then threw them into Tartarus, which is represented as a mountain in the movie, but in the old myths was a cave at the center of the Earth with minor similarities to Hell, to be imprisoned for all eternity. Epirus Bow doesn't exist in any mythology, but it might be inspired by the bow of Heracles aka Hercules which was used by the Greeks to defeat Troy in the great war. (Epirus was a Greek kingdom in what is now Albania, and was home to King Pyrrhus of "pyrrhic victory" fame.) The movie's version of Theseus might be partially inspired by Heracles aka Hercules, a son of Zeus who was the only hero to ascend to Olympus to rule with the Gods. Phaedra was the sister of Princess Ariadne, and the second wife to Theseus. She was not a seer. Acamas was the son of Phaedra and Theseus, but wasn't a seer either. Theseus was murdered in his old age, but not in a battle and he did not ascend. Hoplite was the ancient Greek title for soldiers in ancient Greek states. Heraklion is a capital of the Greek island of Crete and there were never any military formations with that title. In the Theseus story, Crete had its capital at Knossos, and its ruler was cruel King Minos (father of Ariadne and Phaedra), who was the basis for the movie's Hyperion. Sibyl was the ancient Roman title for prophetesses. There were twelve Olympian gods and goddesses and they were immortal and unkillable as well.