Director Zach Snyder originally wanted Brad Pitt for the role of king Leonidas but Pitt turned down the role due to other film commitments (ie Mr and Mrs Smith (2005)
Gerard Butler had 17 helmets made for him, each representing a stage of degradation through out the battle.
The Battle of Thermopylae is often taught to military strategists as an example of the advantages of training, equipment, and good use of terrain as force multipliers.
The script demanded that most of the male cast spend the majority of their screen time bare-chested, as per Frank Miller's original graphic novel. To adequately present themselves as the most well-trained and marshaled fighting force of the time, the entire principal cast underwent a rigorous 8-week training regime organized by Marc Twight, a world-record-holding professional mountain climber. Actors never repeated the same exercise twice, preventing the body from adapting to any one type of exertion. Gerard Butler has said that the training was the most difficult thing he has ever had to do in his life. When it was over, Twight admitted that he pushed the actors as hard as he's ever pushed anyone before, including himself.
Leonidas had an older half-brother Cleomenes, and an older full brother, Dorieus. Because Leonidas was third in line to the throne, he was sent to agoge, the traditional school and warrior training for Spartan youth. However, when Cleomenes ascended to the throne, Dorieus left Greece in a jealous rage. Later, following the Persian attack at Marathon, Cleomenes, suspected of insanity, fled into exile, leaving Leonidas as the remaining heir. Leonidas was thus one of the few Spartan kings to have ever undergone the notoriously harsh training of Spartan youth.