Ewan McGregor's 30-plus films have seen him play all sorts, from a light-fingered heroin abuser to a light-sabred Jedi knight.

With the exception of his cameo in 2010's Nanny McPhee Returns, however, he has never played a father – until now, that is. The 41-year-old Crieff native is back in cinemas with The Impossible, a harrowing survival story drawn from a family's experience in the 2004 Asian tsunami.

"I have been a father for 16 years and yet in my work I have not really explored parenthood," says McGregor when I meet the filmmakers ahead of The Impossible's world premiere at the Toronto Film Festival. "That was one of the reasons I was drawn to the film in the first place. Even though it is set against this unbelievably horrendous, real tragedy, it is a look at the relationship we have with our kids and our family; that unique love we have for our children."

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McGregor has four daughters of his own – with his wife of 17 years, Eve Mavrakis – while his character on screen has three sons. The actor and co-star Naomi Watts play a couple who are holidaying on the west coast of Thailand when disaster strikes just after Christmas, taking the lives of more than 300,000 people, with in excess of 5000 lost in Thailand alone. Did the actor think about his own family when shooting the film?

"I wouldn't make parallels with my family because I have never been in a situation like that with my kids," he says, "and let's hope that I never am. But I had three phenomenal young actors playing my boys in the film, who became just like my boys, in a way, and we shared so much."

The film's opening sequence, a heart-pounding eight minutes where the Indian Ocean swallows the luxury hotel in which the family resides, was shot on location in Thailand. "And out there during rehearsal we were getting to know each other and we did create a bond like a family," McGregor adds. "So I didn't have to utilise thoughts of my own kids. I just thought about my family in the film."

The film is directed by Spaniard Juan Antonio Bayona – best known for his debut movie, the 2007 chiller The Orphanage – and is based on the experiences of a real Spanish family, the Alvarez Belons, though in the film they're played as English speakers. The family's experience is extremely distressing, and Watts has already earned a Golden Globe nomination for her performance as a mother who overcomes so much in a bid to save her children.

"It was a strange film to make for lots of reasons," continues McGregor, "the main one being that it is a true story and there was a real disaster and thousands of people lost their lives. Thousands lost loved ones. You carry that with you all the time. And we should, because it is our responsibility to respect that and to respect all those people." The tsunami had affected many of the Thai crew who worked on the film, he explains, "and you carry that responsibility and you never ever want to feel that you are using it for the purposes of the movie, but yet you are making a movie".

McGregor says he felt no ill will from the locals. "The Thai people have bounced back. It is a place where people go on holiday. They want people to be enjoying themselves. People seem to me to have a very healthy memory about the tsunami and what it meant, but also they want things to move forward and they want to rebuild their country. And I felt that by looking at this one family's story, it might help me understand the tsunami in a much broader sense. I think the numbers are so horrendous you can't wrap your head around it, but by focusing on one story you get some kind of understanding."

The real-life family were closely involved with the production, forging a strong relationship with Bayona and revisiting many of the places where they had suffered and borne witness to such terrible destruction.

"They arrived back there and it was extraordinarily emotional for them," says McGregor. "They came back and were faced with the resort as it had been after the water had hit. The boys are quiet about it and the father, Henry, is a very quiet man, a gentle man, a very nice guy, but you weren't able to read his emotions.

"It struck them all in very different ways. Maria is a very open woman, an emotional woman, and she talks very beautifully and eloquently about how she feels about that experience in her life. She very much wears her heart on her sleeve."

Maria Belon, who has a story credit for the film, insists that she "needed to tell this story, although I was unable to do it by myself - In certain ways, what I experienced during the process of making the movie was the same I had during the tsunami. I thought we were not going to make it, but we did and that is fantastic."

It is the Maria character who suffers most on screen, and English-born Australian actress Naomi Watts summons a magnificent performance as she's physically and emotionally battered by the disaster. The actors spent a month working in a huge wave tank to capture the effects of the tidal surge. The results are more than convincing. Indeed, the actress may well scoop a second Oscar nomination (her first was for 2003's 21 Grams).

Meanwhile, 16-year-old English actor Tom Holland, the star of Billy Elliot the musical, also shines, playing the Maria character's eldest son, with whom she spends the majority of the film.

"It got to that point where we all got so close that Naomi was my real mum on set as well as in the film," remembers Holland, who'll be seen next in director Kevin Macdonald's literary adaptation How I Live Now. "We spent so much time together."

For Watts, 44, who has played a mother on screen many times, the experience was traumatic. "It was the most challenging thing I've done, physically," recalls the King Kong and Eastern Promises star.

"The reputation of water being difficult to shoot with is absolutely true and this was much more difficult that I ever imagined.

"And also the underwater stuff was even more difficult, and extra claustrophobic, and you are running out of breath and so on. I know it doesn't compare to what people actually went through, but it was still hard work. I am not the same age as Tom Holland, and it was hard on the body."

The flood was shot with two units and a tank was employed for the water sequences, measuring 100m x 80m, while Watts was anchored to a chair that spun her round, fully submerged, for a few seconds at a time before rising to let her breathe. Things didn't always go to plan, however.

"Once we had a technical problem with the chair that they used to submerge me," recalls the actress, who will return to screens later this year in director Oliver Hirschbiegel's Diana, the eagerly awaited biopic of the Princess of Wales, "and they weren't able to shut it off."

Watts was trapped underwater. "I couldn't get out and was really struggling for breath," she says. "It went further than we'd planned and it was really frightening."

Watts has already endured one bad experience with water, as a teenager. She was holidaying in Zimbabwe at the time and was caught in a rip tide.

"There are several fatalities on those beaches," she says of the experience, "but we didn't know that and we were tourists just arriving and walking into something that we did not understand.

"I was not a strong swimmer and I did not understand rip tides at all. It was a big moment that we all remember. My whole family was there."

Somehow her feet found the sand and she was able to crawl out of the sea. "It has given me a fear of water, though," she adds, "and that only helped with The Impossible. There are times in the water when I am not acting at all -"

The Impossible (12A) opens on Tuesday.