Naomi Watts has won a Screen Actors Guild Award, but almost fell flat on her face while walking to the microphone for to accept the award.

The Australian actress and her cast in Birdman won the SAG ensemble award, upsetting the favourite Boyhood, at Sunday's ceremony at Los Angeles' Shrine Auditorium.

The ensemble prize is the SAG equivalent to the Academy Awards' best picture trophy and boosts Birdman's campaign at next month's Oscars.

"Oh my God," Watts laughed after recovering her footing on stage.

"Almost tripped."

The slip continued an up and down night for Watts, who was nominated earlier in the ceremony for her supporting role as a pregnant Russian prostitute in the film comedy-drama St Vincent.

That award went to Boyhood's Patricia Arquette.

English actor Eddie Redmayne, who plays wheelchair-bound theoretical physicist Stephen Hawking in The Theory of Everything, scored an upset win over Birdman's Michael Keaton for best actor.

Redmayne's win turned the Oscar best actor race into a thriller, with the Englishman and Bradley Cooper, bolsted by his $US200 million-plus box office hit American Sniper, making a late charge at Keaton.

Keaton didn't appear fazed, as the jubilant Birdman cast won the ensemble prize.

"This is very, very exciting," Keaton said on stage with Watts, Ed Norton, Emma Stone and Birdman's other cast members.

"It is the ultimate team sport, what we are doing."

Best actress was won by Oscar favourite Julianne Moore for Still Alice and supporting actor JK Simmons continued his march to the Academy Awards with a win.

Australia can also claim a SAG win with the stunt team from the Australian-shot, Angelina Jolie-directed Unbroken claiming the outstanding action performance by a film stunt ensemble.

The 21st SAG Awards are voted on by more than 100,000 actors.