It isn't known exactly what character Sullivan Stapleton will portray in 300: Battle of Artemisia, which takes place before the events depicted in 300. We reported in December that actress Eva Green is in talks to portray Artemisia, although it isn't known if that deal was ever finalized.
Ironically enough, Joel Edgerton, who starred in Animal Kingdom alongside Sullivan Stapleton, was also in talks to star in 300: Battle of Artemisia, although he dropped out to join the Untitled Kathryn Bigelow Project.
300: Rise of an Empire was released March 7th, 2014 and stars Sullivan Stapleton, Eva Green, Lena Headey, Hans Matheson, Callan Mulvey, David Wenham, Rodrigo Santoro, Jack O'Connell. The film is directed by Noam Murro.