The role of Themistocles was originally offered to Edgerton, who turned down the role last November while being approached for several other parts.
The Hollywood Reporter initially broke the story of the new casting, but claimed that Stapleton would take on the key role of Xerxes in the follow-up to the 2006 hit. Instead, Rodrigo Santoro, who originated the role of the Persian ruler, is said to be in talks to reprise the role, according to Deadline's sources.
Sullivan will star alongside the already-cast Eva Green, who will take on the titular role of Artemisia.
The film is based on, "Xerxes," Frank Miller's follow-up to his original "300" book. The story takes place over the course of ten years and follows Xerxes as he struggles to become a god.
Snyder will not be returning to direct "The Battle of Artemisia," but he did co-write the screenplay with Kurt Johnstad. Noam Murro will take the directorial reins.
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