The Milk star plays the wizard in Oz: The Great and Powerful, which is based on the books of L. Frank Baum and details the events leading up to Judy Garland's classic 1939 film.
Franco is said to have asked producers if his grandmother Mitzie Verne could make a special appearance, and his request was granted.
A source tells Britain's Mail on Sunday, ""James is very family-orientated. He is very close to his grandma and managed to get her a role in the Oz film. She played a townsperson in the end.""
The movie is due for release in March (13).