IT was just an hour into Boxing Day in the UK and all was quiet.

In Thailand the breakfast-time bustle had begun. Under the Indian Ocean an earthquake was about to send a wave crashing into the shore at the speed of an aeroplane. By the time the tsunami had spent its wrath, more than 200,000 people across 13 countries would be dead.

Eight years on, Spanish director Juan Antonio Bayona revisits those terrible events in the The Impossible. It is an apt title for a film that covers a disaster whose scale was impossible to take in at first.

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Bayona, 37, recalls being at home in Barcelona in Christmas 2004 when the first reports of the tsunami began to come in.

"I was with my family having lunch. I remember there was not much information on the first news. It was a couple of days till we really saw what happened."

The Impossible is the true story of one family's experiences in those first minutes, hours and days, before the press arrived. "What we're trying to explain is what you couldn't see on the news," says Bayona.

The family on whom the film is based, the Alvarez Belons, were Spanish and on holiday in Thailand. In the film they are British, with Ewan McGregor and Naomi Watts playing the parents of three boys. Asked why the nationality was changed, Bayona says the Alvarez Belons were not keen on the family being linked to any particular country. "They didn't want to talk about nationalities, they wanted to feel kind of anonymous, universal."

Both McGregor and Watts had admired Bayona's 2007 debut feature, the Spanish-language horror The Orphanage, which made it easier when the director came calling with roles. It also made The Impossible possible.

"After The Orphanage I knew it was now or maybe never. We were coming from such a big success that we tried to find the sources and the money to do this one."

At the core of the story is what happens to Watts's character and her son, played by Tom Holland. Bayona saw about a thousand youngsters but could not settle on one. Then the casting director recalled seeing Billy Elliot on stage in London, with Holland in the title role.

"I first saw him on a videotape and thought he didn't look like Naomi and Ewan," says Bayona, "but there was something special in there so I went to London and did an audition. It was an extraordinary audition, probably the best I've ever seen with kids." Bayona has not been alone in spotting Holland's potential. Next year, the now 16-year-old will be seen in Kevin Macdonald's new drama, How I Live Now.

Though the casting took time, it was nothing compared to the effort required to recreate the tsunami and its aftermath. CGI was an option, but Bayona wanted the scene of the wave hitting to look as natural as possible. Cue real water, 35,000 gallons of it. After developing the scenes for more than a year, six weeks were spent shooting at a huge tank in Spain.

While making the wave seem as realistic as possible, the actors also had to be kept safe. A system was devised whereby they were perched on what Bayona describes as "big flowerpots" on rail tracks, and moved about. Watts, who has been nominated for a Golden Globe for her performance, says of the water scenes: "You couldn't really speak. All you could do was feel and experience."

When it came to extras, Bayona was determined to use local people. "From the beginning I tried to get in contact with as many people as possible who were there, and got to know their stories, their opinions about what was the right thing to do in portraying the story of this family."

By the time the cast and crew arrived in Thailand, Bayona reckons about 90% of the original locations had been rebuilt, so scenes had to be recreated. Revisiting that time must have been difficult for local people, I suggest, particularly in the hospital that was the first reception point for the wounded. The film shows how, despite their own losses, Thais rushed to help foreign visitors.

The strongest feeling was one of pride, says Bayona. "They were very proud of the film being made because that was a way of showing the world how they behaved during those days."

One of the producers of The Orphanage was Guillermo del Toro, director of Pan's Labyrinth and now one of the biggest players in Hollywood (his latest project: writing the screenplays for a little trilogy called The Hobbit). Del Toro has remained a friend and mentor. When it came to The Impossible, he joked that he would pay not to do it.

"He knew how complicated and difficult it was to shoot with water and with kids. But he knows me and he knew I was going to do it. The whole thing turned into an obsession for me – I had to do it."

The main advice del Toro has given has been to say no sometimes until the right film comes along. With a five-year gap between The Orphanage and his new film, Bayona has followed that advice and seen his film through, proving that the impossible can sometimes be possible after all.

The Impossible opens on New Year's Day.