In Stockholm, the sun went down half an hour ago. It’s 3pm. I’ve just completed the Millennium Tour – an hour-long walk around the city where parts of Stieg Larsson’s bestselling trilogy (65million copies sold in 46 countries) were set

The darkness and the medieval beauty of the city, along with the memory of the books’ violence, have unnerved me as I return to the hotel where I’ll be interviewing Rooney Mara; the relatively unknown actress cast as Lisbeth Salander in the upcoming, The Girl With The Dragon Tattoo.

Fans of Larsson (who died suddenly of a heart attack at the age of 50 in 2004, shortly before the books were published) will know what I mean when I say Rooney is “very Salander”. For those of you who don’t, let me explain. Salander is a slight, tattooed, multi-pierced, social outcast deemed legally incompetent by the state. Sitting before me now, Rooney is neither tattooed, nor multi-pierced and seems entirely capable of looking after herself; but she is very guarded. She’s petite and her short hair dyed jet black makes her look even paler. When she shakes my hand, her touch is so delicate, I fear I may have broken her fingers. We start the interview; her arms are folded. She’s prone to one-word answers – and don’t even think about asking her about her boyfriend, Curb Your Enthusiasm director Charlie McDowell. The room is eerily quiet; I’m starting to worry this won’t go well.

After three very successful Swedish film adaptations of the trilogy – starring Sherlock Holmes 2’s Noomi Rapace – were made just two years ago, it was surprising when The Social Network’s director David Fincher announced in 2010 he would make an American version. Apart from one omission, Fincher’s much slicker version takes in every twist and turn of the novel, something which nudges the film towards the two-and-a-half hour mark …but which keeps you spellbound for every minute. And Rooney is brilliant. Emotionally (and physically) naked for much of the film, she deserves an Oscar nod. Maybe that’s what’s worrying her. The 26-year-old’s life is set to change dramatically.

The daughter of New York Giants football scout Timothy Christopher Mara, Rooney began acting at 20 and appeared in the fairly forgettable Urban Legends and Nightmare On Elm Street remake, before landing the role of Mark Zuckerberg’s girlfriend Erica in Fincher’s 2010 Oscar-winning The Social Network. Fincher says he didn’t originally consider her because she was so compassionate and giving in his previous film, he couldn’t see her as Salander. The competition was tough: Scarlett Johansson’s audition nearly clinched it; Ellen Page filmed her own screen test; Carey Mulligan said she was obsessed with getting the role. But, after getting an audition (and deliberately turning up hungover for the screen test), Rooney was officially attached to the project; her long hair promptly chopped off and dyed black, her eyebrows bleached.

As our interview finishes, Rooney’s parting handshake is strong and my suspicion that she’s not difficult, merely uncomfortable with talking about herself, is confirmed. But having signed up to play one of the most famous literary characters of the past decade, opposite Daniel Craig, Rooney’s days in the shadows are well and truly over.

Fans of the trilogy feel passionate about who plays their favourite characters. Are you ready for the world to start seeing you as Salander?

No [laughs]. But I don’t know if that will necessarily happen. She’s so different to me and I’m not going to look like that in my everyday life. So hopefully people will realise we’re two different people.

Did having to undertake such a significant physical change help you get into character?

Yes, it was really helpful. I wanted to do everything; the piercings, everything. At that point, I was already in the character and ready to be even more so. For me, my long blonde hair was weighing me down. I was really happy to get rid of it and just get on with things.

Will you keep your new look?

No! It’s not the most flattering cut. Hopefully whatever film I do next I’ll have to change again. I’d like to look as different as I can in every film that I do.

There is a graphic scene in which Lisbeth is sexually attacked by her legal guardian. How did you prepare yourself psychologically to film it?

I didn’t really. I had already prepared myself physically and psychologically to play the part before it came to film that scene. David [Fincher] is very protective. But I guess there’s only so much protection you can offer someone when you are shooting a scene like that.

The poster [which shows Rooney in character, naked, with Daniel as Mikael Blomkvist, fully clothed] has caused some controversy. Did you think it was an overreaction?

I did. I mean, I can understand it. People feel very protective of this character and it isn’t the way they saw her. But I didn’t agree with them and that’s OK. Everyone is entitled to their own opinion. The thing that I don’t agree with is that people thought it was very exploitative. I didn’t feel exploited. I knew the picture was being shot. I chose to take my shirt off. Before they ran that picture, David asked me if it was OK. It was my choice. I never felt exploited by it and I don’t think Lisbeth would either.

You first worked with David on The Social Network which won three Oscars. Did you get excited about your role as Erica?

Yeah I did, but you know, I worked on it for less than a week. It was a pivotal role but when you only work four days on something you don’t feel a huge part of it. I’m so proud of that film but I don’t even really feel like I was in it.

Interview: Rooney Mara