The Butler is based on the real-life story of Eugene Allen (played by Whitaker, who also starred in Good Morning Vietnam alongside Williams). Allen served as a butler in the White House under eight different administrations.
Williams, 60, joins an already star-studded cast, including Oprah Winfrey, Forest Whitaker, Alan Rickman, John Cusack, Terrence Howard, Cuba Gooding Jr., Jane Fonda, David Oyelowo, Alex Pettyfer, Vanessa Redgrave and Minka Kelly. (Rickman is slated to play Ronald Reagan, while Cusack will take on Richard Nixon. Kelly will play Jacqueline Kennedy.)