The Light Between Oceans star Alicia Vikander has said that playing a pregnant woman was her “biggest challenge”, and that she was concerned she might not be able to pull it off. Speaking at the Venice film festival after the world premiere of the film, which also stars Michael Fassbender and Rachel Weisz, and was directed by Derek Cianfrance, Vikander, 27, said she “always imagined having a family in the future”, but worried that women in the audience might be able to detect her lack of real-life experience.

“I’m not a mother,” Vikander said, “and it was the one biggest challenge in this film. The extreme longing for a child this [character] has is something a lot of women share. She also goes through miscarriages, and I know it’s a subject that is not talked about much – but it’s common and it’s a trauma a lot of people can relate to. I always imagined having a family in the future, but I sit there imagining half the women in the audience thinking, ‘She doesn’t really know what it’s like.’”

The Light Between Oceans is adapted from a 2012 novel by ML Stedman, and set in Australia in the years following the end of the first world war. Vikander plays Isabel, who marries Tom, a war-damaged lighthouse keeper (played by Fassbender). After two miscarriages, they decide to keep a baby they find in a boat, instead of reporting it to the authorities.

Fassbender, 39, likewise drew attention to the film’s themes of fertility and miscarriage, saying: “War and death comes … as a shadow, with Tom; Isabel is full of life, possibility, fertility, the future. And then, having gone through the miscarriage scenario, that they didn’t have a chance to talk to anyone about, the decisions they made were not made in a healthy way.”

Fassbender also related the fate of Franz, the baby’s real father, to the current immigration crisis, saying: “[Franz] goes out in a boat and dies at sea. We see so many immigrants dying at sea today, and how people react to new people coming into the country.”

The two actors, who are reported to be in a relationship after working together on the film, also talked about their reactions to each other during the casting process. Vikander said that when she secured the role, “it meant the world”. “I was such a fan of Derek [Cianfrance]’s films, and then I knew Michael was attached to play Tom, and I thought he was one of the most brilliant actors out there. So it meant I was up for the game, but I was very nervous.”

Fassbender also said that he was “kind of scared” when Vikander got the part. “She was so fierce and hungry. It’s always a great thing to see in an actor who is getting an opportunity, who isn’t that well-known yet. It was obvious where Alicia was headed, but it’s great to see that hunger ... I remembered what it was like for me when I was starting out. That sort of hunger when new, fresh actors come on the scene really makes more established actors up their game. I felt like I had to get my shit together and respond.”

•The Venice film festival continues until 10 September.