"When you've worked on a film for many years, you come to a point where you are trying to find a solution to the financing and [it was] the only way for us. It was also something Thor wanted."
Ronning, 40, and Sandberg, 41, who grew up in southern Norway in a town near Heyerdahl's, had also been inspired by the latter's 1947 adventure and book, which was translated into 70 languages.
"Thor was the big thinker, a visionary man," says Ronning.
The film shows Heyerdahl as determined to make the trip exactly as he believed the South Americans once did, right down to the balsa wood raft and the ropes that held it together. He found an eclectic group of willing men and, as we travel with them for 4300 nautical miles across the still Pacific (actually the Mediterranean, the film was mostly shot around Malta), we hear their tales of war and what they left behind.
As we see in the closing credits, none of the men returned to Norway apart from Heyerdahl, who sacrificed his marriage for the endeavour and continued making epic rafting journeys afterwards.
"This was two years after World War II and times were tough," says Ronning. "There was poverty and sickness - people were exhausted from the war. I think that for many of the members of the crew this was a way out, a way to think about something else, to get away from it all. Interestingly, Thor brought one guy who could navigate and two guys to operate the radio, so he understood the importance of getting the message, the radio telegrams, out there to get the world's attention. I mean, we jokingly say it was the first reality show.
"He fed the world's newspapers and radios as the world was watching these crazy Scandinavians out in the middle of the Pacific on a raft. Everyone thought they were going to die and every week they were writing how they were - 'still here, the sharks are closing in'. It was a very, very big adventure."
If there were troubles among the men as they drifted along for 101 days, Heyerdahl never spoke or wrote about them. As played by Pal Sverre Hagen, he was a nice, sunny person who managed to deflect any animosity from the crew.
"I think Thor was actually a great leader," continues Ronning. "Everyone we talked to said he had the ability to really get people's attention. He was a great motivator and storyteller. I don't think he sold 50 million copies of his book just because people are into migration theories. It's about the adventure and his ability to inspire people.
"He never ever spoke about the negative sides of his expedition. The science was very, very important for him. He got a lot of people very interested in that too.
"With six men on a raft for 101 days we know there had to be frictions and we dramatised around that. There's always a responsibility for a film-maker when you're doing a biopic regarding the subject matter, and people who read the book will always have an opinion of the way they imagined it. But as a film-maker there's also a responsibility. The film runs for almost two hours so the story needs to have logic and an inner dynamic. Of course, we have taken some dramatic licence but, all in all, it's a true story."
What: Kon-Tiki, the story of Thor Heyerdahl's transpacific raft voyage
Where and when: Screens at Auckland's The Civic tonight (7.15pm) and Wellington's Paramount (8pm) as part of the New Zealand International Film Festival's Autumn Events series
Also: Kon-Tiki is in cinemas from May 16.
- TimeOut