To play a mother torn from her husband and sons by the 2004 tsunami, Naomi Watts sure wasn’t going Method.

The British-Australian actress took her partner Liev Schreiber and two sons to the Thailand set of The Impossible, where Spanish director Juan Antonio Bayona recreated the jaw-dropping destruction in a resort area.

She talked through the story with Sasha, 5, and Kai, 4, to get them used to seeing their mother coughing up blood with a heavily bruised face and deep gashes across her body.

“Yeah, it’s not the best way to see mommy, is it?” Watts laughs in an interview with the Associated Press. “They came first time on a day where I had minimal wounds, nothing too much. . . . And then by the third day they came, they were putting the chocolate powder all over me and painting some wounds on themselves and me. So they understood it.

“I know it’s not completely normal. But this is the life that actors live. And we are playing dress-up some of the time.”

In an already critically acclaimed performance, Watts portrays real-life Spanish doctor Maria Belon, who was swept away by the rush of water with her eldest son and treated in a Thai hospital until she was reunited with her two younger sons and husband, played by Ewan McGregor.

The Indian Ocean earthquake and resulting tsunamis on the day after Christmas killed more than 200,000 people across Southeast Asia. But The Impossible — opening Friday — is a disaster movie that narrows its focus to one family’s survival.

Watts, 44, says the role was among her most challenging ever, physically and emotionally, with the added layer of trying to tactfully convey a horrific reality. Belon has joined Watts and the filmmakers at premieres.

“It was a great pressure and responsibility to get it right because of what she went through and how much she suffered,” Watts says of Belon. “And then on top of her story, it was hundreds of thousands of others.”

McGregor, whose character is not injured as badly, says he was impressed by his co-star.

“For 85 percent of the movie, she’s really badly injured, fearful for her life and the safety of her husband and her other two sons, anticipating the fact that she might be dying,” he said. “There’s never a sense of that being on one note. I’ve talked to her about it. I said, ‘It’s an extraordinary achievement that you managed to weave such variety into it and take us on this huge journey.’ ”