Zack Snyder originally wanted Brad Pitt for the role of King Leonidas, due to his performance as another ancient Greek hero, Achllies, in Troja (2004), but Pitt turned it down due to other film commitments.
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.
Gerard Butler had 17 helmets made for him, each representing a stage of degradation through out the battle.
(at around 1h 35 mins) Leonidas' final words to Ephialtes, "may you live forever," are an insult against the hunchback's desire to be a Spartan, whose greatest glory is to die in the battlefield. After the events at Thermopylae, the word "ephialtes" entered the Greek language, meaning "nightmare," or to describe someone as the ultimate traitor.