Naomi Watts was in Los Angeles, on a break while filming “King Kong,” when it happened.

“I was glued to the TV, watching in shock and horror,” she says. When George Clooney called for her help with a TV benefit for the victims, she was there.

Director Juan Antonio Bayona (“The Orphanage”) was home in Spain, “having Christmas with my family,” when word of the tsunami of 2004 arrived. He too, felt the horror, images that he says he could not shake from his memory. And when he heard one family’s story of their experience in Thailand, he became a movie maker with a mission.

“I don’t know how, and I don’t know why, but the moment I heard this story it spoke to me in this very direct way,” he says. “It turned into an obsession for me.”

Bayona’s obsession was to make a movie about the catastrophe, personalizing, telling one family’s true story of tragedy and triumph. “My intention was to show what the news could not — to empathize with these characters, to personalize it by limiting the point of view to only what they saw and knew and when they knew it. I wanted an emotional experience, an emotional journey, not an intellectual one. The tragedy happened so fast, in so few hours, that even those living through it did not have time to think about it. So it had to be very emotional.”

And to get that emotion across, to film “The Impossible,” as his movie was titled, he needed Naomi Watts to play the badly injured mother, trying to take care of herself and her son, not knowing if her husband (Ewan McGregor) and other sons survived the disaster.

“She has a natural instinct for tragedy, a gift for playing tragic women,” Bayona says. The actress “has few equals at conveying physical and emotional extremis,” Justin Chang wrote in “Variety,” where he praised “The Impossible” (opening Dec. 21) as “emotional” and “the most harrowing disaster movie in many a moon.”

The movie-star-gorgeous Watts, 44, would have to cast aside any thoughts of the glamorous world she travels in, and the vanity that draws so many to acting.

“There is a time and a place for vanity, and I knew that by the fifth page of the script that this was NOT going to be that film,” she says. “I welcomed letting go, having the wear almost no clothes — blood and wounds and scratches and dirt and very bad hair days. You can’t be thinking, ‘We must put the wound over here because that highlights my cheekbones’ This is not that film.”

What Maria, her character, offered, was the chance to play a heroine with conflicting agendas. She is a woman, clinging to life, a mother, struggling to live long enough to see that her son is safe, a woman who must put aside the grief for her missing family and her fear for her own life and pass along one last life lesson on to her son.

“She is a hero in the truest sense,” Watts marvels. “What surprised me was how she took everything down to the purest instinct of herself. Her priorities got into alignment, and she was very sure of every decision she took. Every moment, she was utterly in the here and now. It’s fascinating, the way she was able to access that clarity, that purity of purpose, when she needed it. She trusted her instincts every time she had to make a decision.

“She couldn’t live beyond that space she was in. Her other kids are probably dead? Her husband too? For her, and her son, that wasn’t their story in this moment. It was in them. They realized it. But they couldn’t go there. If they do, they’re weakening themselves and reducing their chances of survival. She needed to survive and needed her son to survive.

“I don’t think we have that kind of clarity in our lives – EVER.”

Bayona can’t help getting choked up when he recalls Watts in her most wrenching moments, a “woman who has to decide, moment by moment, as she is dying, what would be her last act in her life, what she would teach her son. What she chose was a lesson in humanity, that as dire as their situation is, they must take a moment to try and rescue this other kid. I think that lifted her in her son’s eyes, and in the audiences, to the level of hero.”

He set out to tell one family’s story of survival on an epic canvas, a movie that would require vast, devastated sets and convincing special effects. But what Bayona was left with, after doing all the research, was a renewed faith in humanity, something he decided that “The Impossible” must show.

“The most beautiful ideas that emerged from our research had to do with how kind people were,” he says, getting emotional again. “People who had lost everything were the most giving. It’s a difficult film, with some very uncomfortable images, but I hope it is a very positive way of seeing humanity. The tsunami was awful, but people’s response to it was gentle and full of kindness.”

(Review of “The Impossible,” the best picture of 2012)