Lena Headey has been cast as the queen opposite Gerard Butler in director Zach Snyder’s “300,” which Hollywood Gang Prods. and Atmosphere Pictures are producing for Warner Bros. Pictures. Production begins Oct. 17 in Montreal, where Snyder (“Dawn of the Dead”) will shoot actors in front of a green screen, relying on all CG backgrounds. Based on Frank Miller’s graphic novel of the same name, the film is about the real-life battle of Thermopylae, in which King Leonidas (Butler) and 300 Spartan warriors fought to the death against the massive Persian army. According to lore, their bravery led to Greece uniting and giving rise to democracy.