Cast caught in a wave of emotion shooting The Light Between Oceans

From left, Derek Cianfrance, Michael Fassbender and Alicia Vikander. Davi Russo / DreamWorks

"I feel like I'm a documentarian of fiction," says Derek Cianfrance, the American writer-director who established himself as a forensic chronicler of human bonds with the gritty romantic drama Blue Valentine and the thriller The Place Beyond the Pines.

His latest feature, The Light Between Oceans, is an adaptation – of M L Stedman's New York Times and USA Today bestseller, which has been translated into more than 35 languages – but like his previous films, it explores the ­nitty-gritty of a relationship with intimate precision.

At the film’s centre are haunting, committed performances by ­Michael Fassbender as Tom, a traumatised First World War veteran who takes up a job as a lighthouse keeper on an isolated island off the Australian coast, and Oscar-winner Alicia Vikander as Isabel, the optimistic young woman who becomes his wife.

When she suffers two miscarriages, their Edenic existence starts to lose its charm. Then fate sends the couple a baby, washed-up on the shore in rowboat. They keep her as their own, but an encounter years later brings an end to their fragile happiness.

Cianfrance shot the film on an isolated peninsula in New Zealand, where the howling wind kept cast and crew awake at night, adding to the intensity of the ­story.

“It made us a little crazy,” says Cianfrance. “In the middle of the night, the wind would come. It was so violent. You’d then get up at five, not having slept, and all of a sudden you’re here to make a romance movie, and you’re ­unhinged – you’re going crazy.”

“The wind was just relentless,” Fassbender says. “But I understood how the previous lighthouse keeper could have gone nuts. It made sense to me how maddening the wind could be.”

The X-Men star was initially hesitant about living and working in the same location, as he worried he wouldn't have a retreat where he could recharge his emotions each night before the next day's filming.

“It seemed to me that I needed to get away because it was so intense, what we were going to be doing.”

However, persuaded by ­Cianfrance’s passion for the ­project, Fassbender agreed to give it a go.

“He came out one night, and that turned into two nights – and eventually that turned into five and a half weeks,” says the director. “By the end, I had to pull them, kicking and screaming, from the place.”

The Light Between Oceans doesn't just chart the love story of Tom and Isabel, but also of Fassbender and Vikander, who became a couple during filming. Perhaps shooting a scene in which she shaves off his moustache with a cut-throat razor was the moment they fell in love. We'll likely never know, because they've made a pact to keep their private lives private, but the scene feels real, unguarded and true, illustrating perfectly the authenticity that Cianfrance strives for in his work.

“I always think about chefs,” he says. “They have to make, like, the same meal 300 times a night – it’s about consistency. As a film director, I don’t feel like I need to be consistent at all. I feel like I have to get one moment that can’t be replicated. So that’s my whole quest on set with actors, to find a moment that can’t be repeated.”

For her part, Vikander says that not being a mother made her ­worry that people wouldn’t believe that she could know what it feels like to have a child, and therefore wouldn’t believe her performance.

On the other hand, fear is partly what motivates the in-demand Swede, who won the Best Supporting Actress Oscar this year for her role in The Danish Girl.

She used to do whatever parts came along, grateful for the work. Now she’s in a position to choose her projects, she says she looks for roles “that are very far away from anything I have done before and that I feel quite terrified taking on, because I don’t know where to start. The fun part is then the journey of finding that character”.

This made Cianfrance the ­perfect collaborator.

“The first time Michael and I met with him,” Vikander says, “he said he expected his actors to surprise him. It was a challenge, and a wonderful thing.”

The raw emotion she produces is as primal and roiling as the epic ocean backdrop against which Cianfrance shot Tom and Isabel’s story, to make their experiences seem simultaneously insignificant and momentous.

The result is shattering, and heartbreaking.