Cast member Michael Sheen poses at the premiere of ''Midnight in Paris'' at the Samuel Goldwyn Theatre in Beverly Hills, California
Cast member Michael Sheen poses at the premiere of ''Midnight in Paris'' at the Samuel Goldwyn Theatre in Beverly Hills, California May 18, 2011. The movie opens limitedly in the U.S. on May 20. Reuters

Michael Sheen is stepping into Paul Bettany's sex-researcher shoes.

The Twilight Saga and Frost/Nixon actor has been cast in the title role of Showtime's upcoming pilot Masters of Sex. Sheen is replacing A Beautiful Mind star Bettany, who withdrew from the project in January for undisclosed reasons.

Sheen will play William Masters, who together with Virginia Johnson pioneered the field of sex research in the 1960s. The actor will also serve as a producer on the project.

Lizzy Caplan (Mean Girls, Cloverfield) will play Johnson, Showtime also said Thursday.

Production on the Sony Pictures Television pilot will begin in New York and March. Michelle Ashford, Judith Verno, Sarah Timberman and Carl Beverly are executive-producing.

The project is based on the Thomas Maier book Masters of Sex: The Life and Times of William Masters and Virginia Johnson, the Couple Who Taught America How to Love. Ashford, Timberman and Beverly wrote the script.

Sony New York Sony New York