With the Oscars only two weeks away, the acting awards gave a major boost to Roberts, Hollywood's Pretty Woman, who, while adored by audiences, has never won an Academy Award.
"This is freaky," Roberts said in her SAG speech, at times choking back tears. "If this was a prom, this would be like the best school ever to go to. But it's not, so I'll be serious, in my own special way."
Backstage, Roberts told reporters that the real-life Erin Brockovich - who took on a major California utility and won a pollution case - was an inspiration.
"I think that she's remarkable," Roberts said. "She really is the perfect example that we are what we choose to be. We are as effective as we are tenacious."
Roberts has the inside track for an Oscar. But just about everything else at this month's Academy Awards ceremony seems up for grabs.
SAG's award for a movie's ensemble, generally viewed as the guild's equivalent of a best-picture prize, went to the cast of Traffic.
Copeting for best picture at the Oscars are Chocolat, Crouching Tiger, Hidden Dragon, Erin Brockovich, Gladiator and Traffic.
Unlike last year, when American Beauty dominated earlier honors and went on to prevail at the Oscars, this awards season is fairly fragmented.
Judi Dench of Chocolat won SAG's supporting-actress award, but Kate Hudson of Almost Famous took the Golden Globe in the same category.
Benicio Del Toro of Traffic has won both a Golden Globe and a SAG prize. But the Globe was for supporting actor, while the acting guild gave Del Toro the lead-actor trophy.
Tom Hanks of Cast Away, who won the lead-actor Golden Globe for drama, was among those losing out Sunday to Del Toro.
Come Oscar night, Del Toro will be back in the supporting-actor lineup, where his competition will include Albert Finney, who won the SAG honor in that category for Erin Brockovich.
Backstage with Michael Douglas, Catherine Zeta-Jones, Don Cheadle and other co-stars of Traffic, Del Toro credited the entire cast of the drug-war drama, which had about 100 speaking parts.
"I feel like if they put me in the driver's seat, I'm going to take the wheel," Del Toro said. "But I see this picture as an ensemble, and I'm very proud to be in this room. Some of the best actors in America are in it."
Lee appears to have the edge for best director after the guild win and his earlier triumph at the Golden Globes. Since the Directors Guild first gave its award in 1949, only four recipients have failed to win the Oscar.
But Oscar voters are a much broader group, including acting, sound, editing and music branches. The tastes of the full academy may not necessarily jibe with a single group such as directors.
"I don't really believe in film competitions or that you can compare filmmakers," Lee said after his Directors Guild win. "But I do believe this recognition is a great honor."
Besides Scott, Lee's Oscar competitors are Stephen Daldry of Billy Elliot and Steven Soderbergh, who is nominated for both Erin Brockovich and Traffic. The Directors Guild also had nominated Soderbergh for both films.
Before Saturday's ceremony, Soderbergh said he would not offer academy voters any guidance on which of his movies he would prefer them to support. Some in Hollywood feel that could cost Soderbergh an Oscar because his votes might be split between Erin Brockovich and Traffic.
Others say Soderbergh's two nominations will not cancel each other out because Oscar voters generally support the one achievement they think is most deserving.
"I don't see why this is any different, just because he's nominated for the two movies," said Scott Greenstein, chairman of USA Films, which released Traffic.
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