Butler will star opposite Fiennes, who is making his directorial debut, in the contemporary reworking of the Rome-set political and family drama. The script has been penned by John Logan (The Last Samurai).
According to The Hollywood Reporter, the 300 star is cast as Tullus Aufidius, commander of the Volscian army. William Hurt, Eddie Marsan, Jessica Chastain and Vanessa Redgrave have previously been confirmed for the project.
Shooting begins in Serbia next March.