A harrowing, realistic recreation of the 2004 tsunami that destroyed parts of coastal Asia and claimed the lives of thousands is at the center of "The Impossible," and its filmmakers were mindful of the environment while shooting it in Spain and Thailand. The seawater used to fill giant tanks was darkened with food coloring and filled with debris made of organic materials, then treated and recycled, reports producer Belen Atienza. "We made sure there was nothing to harm the actors or the environment. We worked with the government in the area on that."

The true story of a family vacationing in Thailand that was separated when the tsunami hit, "The Impossible" stars Naomi Watts and Ewan McGregor and opens in New York and Los Angeles on Dec. 21 before a nationwide rollout Jan. 4. "There was a huge challenge in the technical aspects of the film, but for me that was exciting," says director A.J. Bayona, who mixed effects techniques — practical, CG and miniature models — "so the audience doesn't know what they're watching. But the real challenge was to portray the story of the people that were there and be respectful of that."

At first, Watts was reluctant to sign on for several reasons. Loath to do another role as emotionally and physically demanding as "King Kong" again, and she felt that "with so many lives lost and affected it feels wrong to make a movie about it." Plus, she isn't a good swimmer and had a fear of water ever since nearly drowning in a riptide in Bali years ago. But the fact that Bayona was directing and reading the screenplay changed her mind. "Five pages in I knew I wanted to do it."

That meant six weeks filming in the water, acquiring bruises, scrapes, and "a horrific hacking cough" she's still trying to get rid of, but the rewards were worth it. She treasures her connection with the woman her character is based on, Maria Belon, who as a producer on the film was available to her. Watts, who will play Princess Diana and Marilyn Monroe in upcoming movies, is quite aware of "the responsibility you have playing real-life people especially when people know them and have their own idea of who that person is. With Maria there's a different kind of pressure. It didn't matter how I walked and talked and looked because nobody knew of her but it mattered so much in terms of telling the story in the most truthful way, given the amount of suffering that she and her family went through."

Watts has already received Golden Globe and Screen Actors Guild nominations for her performance, and the Oscar buzz is building. "I try not to think about that," she says. "I've been down that road before."

McGregor, who plays her husband Henry, escaped having to work in the water but was no less affected emotionally. The father of four daughters, who hadn't played a dad before, tried to block out thoughts of his own kids in the hellish scenario posed by the movie. "You're always acting from two places, your imagination and your experience in life," he says. Having become close to the three young actors who played his sons, "I was able to use that, how desperate it would be to be in that situation."

McGregor's future film slate includes the 3D fantasy "Jack the Giant Slayer," in which he plays the head of a security detail guarding a princess, and "August, Osage County," the John Wells-directed adaptation of the Pulitzer Prize-winning play, with an all-star cast including Meryl Streep, Julia Roberts, Chris Cooper and Abigail Breslin.

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