The piercings, tattoos and jet-black hair are long gone, but there's still something of Lisbeth Salander about Rooney Mara.

The 27 year-old actress, who leapt to fame playing Stieg Larsson's fictional heroine in David Fincher's adaptation of The Girl With The Dragon Tattoo, arrives for our interview all in black. Shirt, jeans, jacket and boots – it's as if she's raided the wardrobe of the bisexual hacker Salander, or can't let go of the role that won her an Oscar nomination for Best Actress last year.

We're sitting in a Berlin hotel room – Mara sipping on a fruit tea, her hair wrapped up in a bun. "I feel that my life is really simple," she tells me, relief in her voice. "I'm still not recognised." That surely is about to change, with Mara starring in Side Effects, Steven Soderbergh's latest film (and possibly last, if he keeps his word about his retirement). A thriller set in the world of prescription medicines, Mara plays Emily Taylor, a New York spouse who is put on a new depression-battling wonder-drug, only to experience some deadly consequences.

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Dealing with a very contemporary issue – the over-use of mood-stabilising drugs in western society – for Mara it felt all too familiar. Raised in New York, where her father is vice-president of player evaluation for NFL team the New York Giants, it was all around her. "Definitely growing up, I knew a lot of kids who were on prescription drugs, for sure," she says. "It's also really easy to get a prescription. It's really easy to fake needing one, because everything is online. You just look up all the symptoms. People always do it."

For Mara's first role since Dragon Tattoo – one that offers far more scope than playing the antisocial Salander did – she read the script the day she was nominated for the Oscars, and had 12 hours to decide. "It was really a no-brainer," she says. "I really wanted to work with Steven, first and foremost. I knew he was gonna be retiring and it was probably my last chance to do that." So is it true? Is the director of Traffic, Ocean's 11 and Erin Brockovich really on his way out? "He hasn't given us any reason not to believe [him]," she replies, with a shrug.

As confident as she is on screen, Mara is more insular in person. Despite a career that's seen her go from the remake of A Nightmare on Elm Street to Fincher's The Social Network, she doesn't yet feel "comfortable" rubbing shoulders with the Hollywood elite. "I definitely feel like an outsider – put it that way. It can be a very isolating job. It's not like you always feel like you're in this big community.

"I think other people do feel that way, [those] who've been in the club longer, but I feel like an outsider still. I feel like I shouldn't be here."

Unsurprisingly, she felt exactly this way on Oscar night – competing with Michelle Williams, Viola Davis, Glenn Close and eventual winner Meryl Streep. "Even though I knew I wouldn't win, it was still very nerve-wracking. You're on display." She didn't go to the bathroom all night, perhaps wracked by the fear that she might – just might – win. "Because I feel like an outsider and don't feel like I deserve to be there yet, I would've been horrified to go up. I would've been so scared. Not that I wouldn't have been grateful – I just would've been terrified."

One of four children, Mara's older sister Kate is also an actress (and has just recently been cast in Fincher's Netflix-broadcast remake of the BBC parliamentary drama House of Cards), though the two are very different. "I think it comes easier to her. She's a little bit more open and friendly than I am. She's the likeable one." Still, after graduating from high school early, Mara joined the Travelling School, an open-learning environment that took her to South America for four months, across Peru, Bolivia and Ecuador. "Getting out of the bubble of where I grew up and going out and seeing the world changed my life," she admits.

Having also established her own charity in Kenya, Faces of Kibera, underneath her rather shy persona, there is a steely determination to Mara, even if she can't quite adjust yet to how in demand she really is. She's just worked for Being John Malkovich director Spike Jonze, on his new future-set film, Her. And, having shot indie movie Ain't Them Bodies Saints with Ben Foster, she also spent two-and-a-half months with Terrence Malick on his as-yet-untitled new film, rumoured to be set to the backdrop of the Austin music scene. "It was the most unique experience I think I'll probably ever have in my career," she says. "I can safely say that."

There's also still talk that she will reprise her role of Lisbeth Salander for sequel, The Girl Who Played With Fire. "Hopefully they will do it," she nods. "It's definitely something I'd want to do." For the moment she's planning to take some time off; originally intending just to make Side Effects and the Malick movie, until the other two popped up, last year's furious workload has left her spent. "I hadn't planned on doing four films. I think that's a lot," she says. "I don't want people to get used to seeing me and annoyed by my face." There's little danger of that – at least for now.

Side Effects opens on March 8