The Scottish actor - who is currently promoting 'The Bounty Hunter' with Jennifer Aniston - is heading to Belgrade to start shooting the movie, in which he plays rebel leader Tullus Aufidius, and he is currently bulking up.
He said: "Tullus is totally fearless and I'm getting in shape and having fight training. War's not supposed to be fun, but I'm looking forward to making the film with Ralph."
The story - which was originally a tragedy written by legendary English playwright William Shakespeare - tells the tale of a hero of Rome who is banished from the city only to return with its sworn enemy in an effort to avenge his honour.
Speaking about his character, Gerard told Daily Mail columnist Baz Bamigboye: "I'm playing Tullus Aufidius, the general of the Volscian rebel forces. They're the underdogs and Tullus is Coriolanus' sworn, hated enemy - but Coriolanus respects him.
"They have shed blood on the battlefield and there's an admiration they have for each other."
As well as directing the movie, Ralph plays titular character Coriolanus, while recent BAFTA Academy Fellowship honouree Vanessa Redgrave also stars.
The movie is still in production and does not yet have a preliminary release date.