Jessica Chastain: I will not admit my age because I'm an actress

When Kathryn Bigelow calls, most people drop whatever they're doing; in Jessica Chastain's case, that meant abandoning plans to star in Iron Man 3.

If Iron Man would have been a fun ride, then Zero Dark Thirty took her to the depths of hell – or at least to a squalid Jordanian prison where she was humiliated by prison guards, an experience that ultimately delivered an Oscar and BAFTA-nominated performance. She has already taken home a Golden Globe.

As recent evidence has indicated, it was actually a woman who helped to locate Osama bin Laden after a decade-long hunt for the 9/11 terrorist. And who better to play this brilliant but elusive female CIA analyst than this highly accomplished actress, who has conquered Hollywood while, at the same time, managing to preserve her own secrets.

"I learned as much as I could about her. She's a real woman, but we wanted to protect her because she's still a member of the CIA," says Chastain, referring to her character Maya, a version of the same agent referred to as "Jen" in former Navy SEAL Mark Owen's book, No Easy Day, an inside account of the raid that killed Bin Laden.

The film's title is actually a military term for 30 minutes after midnight, but it also refers to the darkness and secrecy that cloaked the entire manhunt.

"I didn't want to play her in a way that would expose who she is. We didn't want another situation, like when Valerie Plame was outed and her career was ruined as a CIA agent," adds the actress, who likewise prefers to keep much of her own identity private to the point of not even revealing the Northern California town where she was raised, one of five children, by a firefighter father and home-maker mother. Nor does she confess to her age, although she is widely understood to be 35 years old.

"I will never say my age because I'm an actress, and I want to play different ages," she demurs when we meet at a hotel in Los Angeles.

Almost as covert as the character she portrays in Zero Dark Thirty, she slinks unannounced into the windowless, unmarked basement room where I have awaited her arrival, interrupted every few moments by misdirected actresses attending a casting call in a neighbouring room.

"What did they look like?" she asks, blue eyes sparkling in freckled skin as I describe the scenario. "They mostly looked like models," I tell her in all honesty.

"That sounds like my life in LA, five years ago. I would show up at auditions and be like, 'What's going on?'" sighs the strawberry blonde, classically trained thespian, who exploded into Hollywood in 2011 with six films, including Terrence Malick's The Tree of Life, Al Pacino's Wilde Salome and Ami Canaan Mann's Texas Killing Fields.

If her arrival threatened to go unnoticed by all but independent movie audiences, then her break-out role in that same year's people-pleaser, The Help, cemented her future, with a best supporting actress Oscar nod.

And, as she said in her Golden Globe acceptance speech last Sunday, she has worked very hard for a long time, mostly in theatre, before receiving any recognition.

Having already played a spy in 2010's The Debt, she had strong ideas about her performance in Zero Dark Thirty: "Most of the time when you see a spy movie with a woman, so much of it is about their feminine wiles – and they're seducing and using sex all the time; but this is a woman who uses her brain."

During the course of filming in Jordan, there must have been many times when Iron Man 3 started to look appealing. "I had a hard time doing the torture scenes. All those scenes were filmed in a Jordanian prison and there was a very dark atmosphere, especially with regard to the way women are treated," she says, recalling a particular incident when soldiers insisted that she walk to the prison instead of being driven. "They don't see women that often. I was like, 'I'm not getting out of this car, how dare these guys', but then you think: this woman had to live in Islamabad and all these places when she was doing this job – and had to experience the same treatment of women where she had no control.

"I got pretty depressed while we were shooting all that, and Kathryn did, too – so we would send each other videos of dogs getting rescued! I would come home to my hotel room, where I had taped up on my walls all the pictures of the terrorists in the movie, and I'd sit there all by myself. I'm on the other side of the world and I would go to my computer, crying about my day, and then I would see a video of this sweet dog being found in a garbage can and being brought to a happy home," she says, sipping thoughtfully on green tea.

Chastain had just begun her studies at Juilliard in New York in 2001 when the World Trade Center was obliterated. "Those few months after 9/11, I lived by the Red Cross building. There was a tank on my street, and there was this whole anthrax scare, where we were all afraid of opening letters. We just didn't know what was going on and I felt so far away from my family and friends. It was really traumatic. And I've actually never been down to the World Trade Center site. I didn't want to see it when it happened and I still don't now.

If most actresses will spill their guts for a Vanity Fair cover in the US, in contrast Chastain revealed very little in her own recent cover, claiming that her life was relatively unchanged.

"I said that because I really believed it at the time," she tells me now, the day after Zero Dark Thirty's Los Angeles premiere. "But after the screening, when we got home, as soon as I got out of the car, there was another car that pulled up behind and someone got out and said, 'Jessica, I'm so sorry, but we couldn't get into the screening', and they had followed me to my house. That's when I'm like, 'What's happening?' I mean, they were very nice, but they were strangers and they had followed me for half an hour, tailing my car because they wanted a photograph with me. That was when I realised: 'My life is different now'.

"The one thing I've always worried about is that, for me, acting makes me feel connected with society, a part of something bigger than I am; a part of mankind. But they say that sometimes what happens when you receive fame is that you're excluded. And I've been really lucky, because for the last year and a half I haven't been. And now suddenly I'm starting to see a change where people are treating me differently. And I don't want to be treated differently."

As clandestine as the character she plays, Chastain has tended to keep her love life under wraps, although this week was rumoured to be dating Tom Hiddleston: "I still want to relate and have conversations with people, but I don't want the balance to shift, so it becomes about a man in my life. Most actresses are forced to talk about the men in their life and I just don't find that interesting," she says before an awkward silence elicits a truth: "I'm very shy when it comes to guys. I like to be wooed, but I'd have to be more outwardly available, I guess."

'Zero Dark Thirty' opens on 25 January

This article appears in tomorrow's print edition of Radar Magazine