EXCLUSIVE: Last Vegas has a love triangle and the guys are fighting over Oscar winner Mary Steenburgen. The actress joins the cast of the CBS Films and Good Universe comedy about a group of old pals who throw a Las Vegas bachelor party for the only one of them who has remained single all these years. Steenburgen will play Diana in the Jon Turteltaub-directed film. The character is a Sin City lounge singer who gets close to the soon-to-be-married Billy character played by Michael Douglas. Diana then becomes the object of both the affections of his and co-star Robert DeNiro’s Paddy character. Playing the character of Archie, Morgan Freeman is also in Last Vegas. Kevin Kline rounds out the cast of the four buddies. Dan Fogelman wrote the screenplay for Last Vegas. Laurence Mark and Amy Baer are producing, with Good Universe’s Nathan Kahane executive producing with Lawrence Grey. Matt Leonetti will co-produce. Production is scheduled to start this fall. Steenburgen, who won a Best Support Actress Academy Award for 1980’s Melvin and Howard, recently starred in 2011’s The Help with Emma Stone, Viola Davis, Sissy Spacek and Best Supporting Actress Oscar winner Octavia Spencer. The actress also made a four-episode appearance on 30 Rock this year Steenburgen is repped by the Gersh Agency and Management 360.