Rachel Weisz of the murder thriller "The Constant Gardener" and Paul Giamatti of the boxing drama "Cinderella Man" received supporting-acting honors.
"Oh, my God, y'all. Sometimes, I can't just shake the feeling that I'm just a little girl from Tennessee," said Witherspoon, who plays Carter during her long, stormy courtship with country legend Johnny Cash. "I want to say my biggest inspiration for this movie obviously was June Carter. She was an incredible woman."
Hoffman, considered the favorite for the best-actor Oscar as Capote amid the author's struggles to research and write the true-crime novel "In Cold Blood," had gushing thanks for his "Capote" co-stars.
"It's important to say that actors can't act alone, it's impossible. What we have to do is support each other," Hoffman said. "Actors have to have each others' backs. It's the only way to act well is when you know the other actor has your back, and these actors had my back, and I hope they know I had theirs."
"Brokeback Mountain" has been considered the best-picture front-runner at the Oscars, whose nominations come out Tuesday, with awards presented March 5. Its loss to "Crash" could prove a speed-bump on the film's path toward becoming the first explicitly gay-themed movie to win a best picture award at the Oscars, but "Brokeback Mountain" has dominated earlier Hollywood honors so it will likely continue to be considered the favorite.
Last year, the wine-country romp "Sideways" won SAG's ensemble prize, while "Million Dollar Baby" went on to earn best-picture.
"Crash" follows the lives of a far-flung cast of characters over a chaotic 36-hour period in Los Angeles.
"This celebrates the definition of what an ensemble is all about. There's 74 of us," "Crash" co-star Terrence Howard said of the film's huge cast.
Weisz won supporting-actress for her role as a rabble-rousing humanitarian-aid worker, while Giamatti was honored as supporting actor for playing the manager of Depression-era fighter Jim Braddock. Both had gracious thanks for their fellow actors.
"I can't imagine a greater honor than being acknowledged by my peers," Giamatti said. "Being an actor is a hell of a thing. It's a hell of a thing. It's up and down. It's great, but I found the best thing about it is hanging around the craft-service table with other actors and crew people, eating doughnuts."
"It's so special to be honored by fellow actors, so thanks very much to the tribe," said Weisz, who also won the Golden Globe supporting-actress prize.