Watts shared Impossible with family

Naomi Watts has revealed she had to mentally prepare her two sons when they visited her on the set of new film The Impossible.

The 44-year-old actress stars with Ewan McGregor and rising star Tom Holland in the true story of Maria Belon and her family, who were holidaying in Thailand in 2004 when the Indian Ocean tsunami struck.

Naomi was joined by partner Liev Schreiber and sons Sasha, five, and Kai, four, while filming on location in Thailand, where she talked them through the story to get them used to seeing their mother coughing up blood with a heavily bruised face and deep gashes across her body.

She laughed: "Yeah, it's not the best way to see mommy, is it? 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."

Ewan, whose character is not injured as badly, said 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."