SAG's lead-acting honours went to Jennifer Lawrence for her role as a troubled widow in a shaky new relationship in the lost-souls romance Silver Linings Playbook and Daniel Day-Lewis as Abraham Lincoln in the Civil War epic Lincoln.
Anne Hathaway of Les Miserables and Tommy Lee Jones of Lincoln won the supporting-acting honours.
"It occurred to me -- it was an actor that murdered Abraham Lincoln," said Day-Lewis, a solid front-runner to join an exclusive list of three-time acting Oscar winners.
"And therefore, somehow it is only so fitting that every now and then an actor tries to bring him back to life again."
It was a brisk, businesslike and fairly bland evening as the actors union handed out honours to a predictable line-up of winners who generally had triumphed at earlier Hollywood ceremonies or past SAG shows.
The SAG cast win came a day after Argo claimed the top honour from the Producers Guild of America, whose winner often goes on to claim best picture at the Academy Awards.
Argo also was a surprise victor two weeks ago at the Golden Globes, where it won best drama and director for Ben Affleck.
The awards momentum positions Argo for a rare feat at the February 24 Oscars, where it could become just the fourth film in 85 years to be named best picture without a nomination for its director.
"To me this has nothing to do with me, it has to do with the incredible people who were in this movie," said Affleck, who also stars in Argo and accepted the SAG prize alongside his cast.
The next playoff round before the Oscars is Saturday's Directors Guild of America Awards, where Affleck, Bigelow, Spielberg and Hooper all are nominated, along with Ang Lee for Life Of Pi.
The winner there typically goes on to triumph with directing and best-picture Oscars, but only Spielberg and Lee are nominated for both the Directors Guild and Oscar prizes this time, throwing the awards picture into a muddle.