NICOLE KIDMAN might have relinquished her anonymity 25 years ago, but she's tried her hardest not to become cynical or jaded in the ensuing years.

"I'm still someone who wants to experience new things. I have friends who say: 'You know, I operate at about a four and I never want to go up to a nine, never want to go down to a one'.

"But I enjoy going up to a nine and jumping around and being incredibly excited - beyond excited - but that means I have to be willing to take the twos," says the 47-year-old, who looks 10 years younger.

And despite Kidman's status in the upper echelons of movie stardom, she doesn't want to become suspicious about people's motives.

"I think for me, unfortunately - or fortunately - I'm still incredibly trusting," she says. "That's just the way I operate, but I choose to be that. I choose to stay very open to people, and I never go into something mistrustful."

Born in Hawaii, Kidman moved to her parents' native Australia as a toddler and trained as a ballet dancer, before edging into acting, landing her first film role aged 16.

Worldwide recognition beckoned in 1989, following her performance in the thriller Dead Calm with Sam Neill, and three years later she married one of the most famous movie stars on the planet, Tom Cruise, after they met on the set of 1990's Days Of Thunder. The pair adopted two children but parted in 2001.

Reflecting on her journey so far, Kidman, who now has two young daughters with country singer Keith Urban, whom she married in 2006, says: "There are times where you think, 'Oh God, I would've loved not to have gone through that', but in the same vein, you've got to go, 'But that's what's made me who I am'.

"That's what we try and teach our children, that life is full of contradictions, and the more we can navigate through the pain, the more we'll understand when there's joy. There's no such thing as consistency."

The question of whether we are the sum of our memories and experiences is one of the themes in her new thriller, Before I Go To Sleep.

Based on the bestselling novel by S.J. Watson, the film follows Christine, a woman who wakes up every day remembering nothing, as a result of a traumatic incident in her past. And then one day, new terrifying truths emerge that force her to question everything, and everyone, around her.

"From the minute I read the script, it got under my skin," says Kidman. "And the book itself has been such a huge hit, when I mention the film to people, they say, 'Oh, I can't wait to see it'."

The movie marks the second occasion she's worked with Colin Firth, who plays Ben, the husband she no longer recognises.

They starred together previously in last year's The Railway Man, and were set to collaborate again in the upcoming big-screen adaptation of Paddington, until Firth dropped out, feeling his voice wasn't right for the famous bear. The pair will reunite, however, to begin filming Genius later this month.

"I love working with Colin," Kidman says fondly. "He's so easy, he's extremely nuanced and yet he's a listener. When you're in a scene with him, he listens and responds; that's the greatest acting."

Oscar-winner Firth speaks highly of her in return. "The way Nicole works is chasteningly authentic. I feel like I have to be at the top of my game. I can't lie. I can't be lazy about it. If I try to sell her a bill of goods in a scene and it's not true, I'll see it in her face. I'll lose her. I've got to convince her," he says.

Kidman is known for her comprehensive research and confesses she took on the subject of amnesia with absolute commitment. "Most of my research was to do with the condition, so that it was plausible, because if I can't feel it and believe it, I'm hopeless," she explains.

The thought of suffering from it is "horrifying", she says. "Someone described it as like losing their soul, because you lose your identity. You lose what you are and that's really chilling, and also very sad."

Kidman is no stranger to emotionally complex roles and says: "At different times, I choose to throw myself into things which are probably extreme, but really, the reason I liked this movie was that there were some really great twists," she says.