Jon Favreau has added another actor to his upcoming sci-fi comic film, Cowboys & Aliens and its “Mad Men” and “Hawthorne” alum Abigail Spencer. According to Heat Vision she’s officially signed on and will be making her big screen debut. She’ll be joining the cast that currently includes Daniel Craig, Harrison Ford, Olivia Wilde, Noah Ringer, Sam Rockwell, Paul Dano, and Adam Beach.

Spencer who’s best known for playing Don Draper’s mistress on “Mad Men” will stay on that amoral path for her role in Cowboys & Aliens. She’ll star as a prostitute named Alice, a former lover of Craig’s character who makes him change his ways. Doesn’t that remind you of Megan Fox‘s role in Jonah Hex? We’re sure Spencer will do a better job though (and look less sweaty while at it.)

Cowboys & Aliens will center on outer-space creatures who land in Arizona circa the mid-19th century intent on enslaving the local population — until a group of cowboys and Apache Indians fight back. Zeke Jackson (Craig) is a very talented gunman and will be the revolt leader.

Cowboys & Aliens is slated to hit theaters on July 29, 2011

What do you think of the latest addition to the cast of Cowboys & Aliens? Are you interested in seeing the film?