Britain's Colin Firth, seen as a front-runner for his role as a stammering monarch in "The King's Speech," and nominees Annette Bening, Natalie Portman, Amy Adams, Nicole Kidman and Jeff Bridges are among those expected at the Los Angeles ceremony.
Some 120,000 actors in the U.S. film and TV industry pick the Screen Actors Guild winners, who are seen as top contenders at the February 27 Oscars because actors make up the largest voting group among members of the Academy of Motion Picture Arts and Sciences.
The guild does not hand out prizes for best movie. But its top prize of best cast ensemble should narrow down a close race for the movie industry's highest honors between Facebook movie and Golden Globe winner "The Social Network," and "The King's Speech," which has a leading 12 Oscar nominations.
Ballet thriller "Black Swan," boxing drama "The Fighter" and lesbian family movie "The Kids Are All Right" are also competing for SAG's best cast trophy.
SAG best actress nominee Portman, 29, who plays an unhinged ballerina in "Black Swan," is thought to be neck and neck with Bening for her performance as a lesbian mom whose partner cheats on her with a man. Both took home Golden Globes this month.
Firth has swept the Hollywood awards season so far and earned an Oscar nomination for his role as shy King George VI -- the father of current British monarch Queen Elizabeth -- who fought to overcome an embarrassing stutter in the 1930s.
Jesse Eisenberg, who portrays Facebook creator Mark Zuckerberg in "The Social Network," is also nominated best actor at SAG, along with 2010 Oscar winner Jeff Bridges for western "True Grit," James Franco for "127 Hours" and veteran Robert Duvall for the independent film "Get Low."
Firth's co-stars Helena Bonham Carter and Geoffrey Rush are nominated in supporting categories, along with three of the cast of "The Fighter" -- Melissa Leo, Amy Adams and Christian Bale.
The guild also honors performances in television. This year the cast of musical series "Glee" will defend its 2010 award for best TV comedy cast, while stylish advertising drama "Mad Men" must fend off a challenge from HBO's lavish new Prohibition-era series "Boardwalk Empire."
Best individual TV nominees include Michael C. Hall ("Dexter"), Steve Buscemi ("Boardwalk Empire"), Julianna Margulies ("The Good Wife"), Steve Carell ("The Office"), Betty White ("Hot in Cleveland"), Chris Colfer ("Glee"), and Tina Fey ("30 Rock").