For a few years when she was growing up in Iceland, Noomi Rapace lived on a farm, but she was too young to get involved with any of the day-to-day labour. So when, on her first day on the set of her new film, Lamb, she had to drive a tractor and deliver a lamb, it was something of a baptism of fire. “I knew I could not f**k up,” she tells me over coffee in a London hotel. “But I didn’t have a lot of adrenaline. There was a strange calmness.” Still, her foray into midwifery left its mark. “The weirdest thing was the very specific smell. It just wouldn’t wash off. It stayed for a week or something.” The Icelandic film, directed by first-timer Valdimar Jóhannsson, was the break-out hit at this year’s Cannes Film Festival. Rapace, 41, plays a woman caught between two brothers, who loosely resemble Cain and Abel: she’s married to one and had previously been involved with the other. It is not the only biblical element to the tale; her character is called Maria, after all. When the childless couple discover a newborn in their barn, they are so excited that they fail to notice there is something odd about it – namely that it is half-lamb, half-human.

When she first read the script, Rapace says: “I had a double-take, and I read it again.” Jóhannsson told her that he wanted the film to be sombre, and to make everything else in its world normal and simple. “So there’s only this one element that is strange,” she says. While Jóhannsson “danced around the subject” of the religious imagery when she tried to discuss it with him, Rapace has her own ideas. “I think Lamb takes place on Christmas Eve because in Iceland, there are many strange and weird energies going around during Christmas. They have this Icelandic Christmas witch called Gryla, who is super evil and catches kids and eats them.” Religion played a significant part in Rapace’s upbringing. “My mother has a cross of Jesus on the kitchen wall, on the table a Buddha. She has Hindu gods, a Native American dreamcatcher over her bed. There’s a Koran and books on Nordic gods. Everything. I grew up in the presence of all religions,” she says. “It was a strange home: always wondering, can anybody hear me?”

Rapace was born in the remote town of Hudiksvall, Sweden. Her mother, a Swedish actress, separated from her flamenco dancer father shortly after she was born. Aged five, her mother and stepfather moved to Iceland, where Rapace became an actress, playing her first non-speaking role aged seven. Aged 15, she left home to go to a Swedish theatre school. Then came a television soap, theatre and an award-winning turn as a teen mother in the Danish film Daisy Diamond. Her life changed in 2009 when she found international fame and received a Bafta nomination for her performance as Lisbeth Salander in The Girl with the Dragon Tattoo. The sequels in the Millennium trilogy were also international smashes and were praised for changing the standard for female roles in thrillers. “I didn’t reflect much at the time,” she says. “I was so deep in it. It took me around a year and a half to do all three films. And then, when it came out, I was in the middle of my divorce. It was so chaotic.”

Born Noomi Norén, she married Swedish actor Ola Norell in 2001. They took the family name Rapace, meaning bird of prey, because they thought it sounded cool. Their son, Leo, was born in 2003. But as her career was taking off, her personal life was in turmoil. “I never wanted to read what they [the press] thought about me or the comments. Because that makes me so anxious, it was a whirlwind and it was kind of mental, everything that happened. My life exploded. Now when I look back, I’m grateful for Lisbeth, but she also messed me up a lot.” After her divorce and the success of The Girl with the Dragon Tattoo, she moved to London, where she still lives with her 18-year-old-son. “It’s my favourite city in the world,” she says. She started to take English-language roles including in Sherlock Holmes: A Game of Shadows and Ridley Scott’s Prometheus. But she would often feel anxious about whether her career decisions were correct.

“It comes and goes. Now it’s less than ever, I would say. For many, many years, I was just surviving, and now I feel like I’m living,” she says. “I love making films, putting things together and connecting people. All the glamour and the glitter, it’s just a loan, just a brief moment, and then you’re back on set, and it’s dirty and muddy and cold.” Watching Rapace in Lamb, it’s true that she seems so much more confident and assured than the actress who appeared in Michaël Roskam’s thriller The Drop, Daniel Espinosa’s Child 44, or Michael Apted’s Unlocked. “I feel like for years I built up this protection around me,” she says. “I was always on the fence, and life was like walking or running on a minefield. And you never knew when the mine would explode. “But now I feel like I’m walking without looking down. I feel like I’m open, and Lamb was the turning point. A lot of things changed in me and I had this awakening. I reconnected with the Noomi that I was before I put on all this protection.”

She is currently filming Django, a 10-episode TV series for Sky that is billed as a reimagining of the Spaghetti Western. “ I wanted to take a break. I had a Skype call with the director and writers who wanted to create a role. Then Nicholas Pinnock texted me for a coffee and I took him to work out with me in a climbing class. As we were leaving, he tells another friend: ‘I’m off to Romania tomorrow and Noomi’s going to play my sister’. And I thought: ‘I haven’t said yes yet’.” She’s glad that she did: “The character is the most beautiful, brutal villain. I dream about her. I told the writers this, and some of my dreams have become part of the script.” The good roles have come at a time when she is more at peace with herself, and more willing to be open about her difficulties than in the past. “I have accepted imperfections and allowed myself to feel sadness, vulnerability and weakness. That’s not weakness. I was raised in a way never to cry, never show weakness. So I became very tough,” she says. “I don’t want to live like that. I don’t want to accept that way of living, and carrying myself, to bleed into my son’s life. I want him to be a human that has access to all emotions, where everything is allowed.”

Lamb is in cinemas from Friday