Clooney and Martin Sheen were among the first stars to sign up to star in the Milk writer's new play, entitled 8, which focuses on the Proposition 8 bill, which overturned the legalisation of gay unions in the state.
Now Pitt has added his name to the all-star cast, which will also feature Kevin Bacon, John C. Reilly and Glee star Chris Colfer. The Moneyball star will portray a judge in the play.
Stand By Me filmmaker Rob Reiner will direct the production, which will be staged at the Wilshire Ebell Theatre in Los Angeles on Saturday (03Mar12). Proceeds from ticket sales will benefit the American Foundation for Equal Rights.
Proposition 8 has since been deemed unconstitutional by a U.S. appeals court.