The drama chronicles the true story of a family's survival in the aftermath of the massive tidal that wave swept across the Indian Ocean and killed over 230,000 people in December, 2004.
Few men and women in the impact area made it out alive, and McGregor reveals some of them were recruited by director Juan Antonio Bayona to star in the movie as a sign of respect.
McGregor explains, "In some of the scenes, Juan Antonio Bayona, the director, decided to surround us with real survivors from the 2004 tsunami, which was very special. There was an energy about that, which was really useful for us... We were surrounded by people who really knew this first-hand experience of a tsunami."
And the Big Fish star reveals he and Watts tried to be as accurate as possible by working with the Spanish couple they portrayed in the film on location at the exact hotel where the family stayed.
He continues, "We shot all of the film in Thailand in Khao Lak, where the family had actually been staying, and in fact, shot in the hotel where they had been staying (Orchid Beach Resort)... We spent a lot of time with them, they're really incredible; it's always a responsibility when you're playing somebody who exists.
"In this case I was working with a really lovely man, Henry, who I liked very much, but our responsibly was mainly to everybody who was there, the people who lost their lives there, the people who lost their loved ones there. It's a terrible tragedy... We tried to hold that respect in our hearts."