Rooney Mara

The film's director David Fincher looked far and wide for his main character.

He flew in actresses from New Zealand to Swaziland and even screen tested Scarlett Johansson and Natalie Portman, but failed to fill the role.

When the casting director suggested Rooney, whom the filmmaker had directed in the opening scene of The Social Network, he resisted – thinking he needed someone who was anti-social – the exact opposite of Erica Albright, the whip-smart, highly verbal girl seen breaking up with Marc Zuckerberg in the film.

But after three months of gruelling auditions, he and nervous Sony bosses became convinced that a virtual unknown could carry the weight of a potentially massive film franchise.

So began Rooney's dramatic transformation into Lisbeth. Her long hair was cut and died black, eyebrows bleached blonde and her body covered with piercings.

"I didn't recognise myself," she said. "The eyebrows were the biggest shock because that really changed my face. But I was fine because I knew it was going to be really useful for getting into character."

To further help the cause she took up skateboarding and kick boxing and had training in dialect and computers, and even went to live on her own for two weeks in Stockholm, where the film was shot.

The work paid off. The award nominations came flooding in, and even the film's weaker reviews singled out Rooney's brilliant performance. It was clear David had chosen the right girl for the job.

Born in New York in 1985, to Chris Mara, a vice president at the New York Giants, and Kathleen Rooney Mara, a part-time real estate agent, Rooney is one of four children.

Due to her family history – her great-grandfathers are Tim Mara, who founded the New York Giants, and Art Rooney, founder of the Pittsburgh Steelers – words like "football heiress" are often used inaccurately to describe the young star.

"I don't have a trust fund," she says in interviews. "I grew up in a little cul-de-sac in the suburbs and went to public school."

Both she and her actress sister Kate – known for roles on 24, American Horror Story and Entourage – caught the acting bug off their mother, who took them to Broadway shows and showed them old movies.

Kate started acting aged 12, but her sister wanted to go to college first, and moved to LA after graduating from NYU to try and make a go of it.

She began auditioning aged 19, and one of her first roles was on Law & Order: SVU. But she was almost put off acting together by taking part in the unsuccessful Nightmare on Elm Street remake.

"I hated it," she says of the experience. "It left me thinking, If this is what is available to me, then I don't necessarily want to be an actress.

"And then I got the script for The Social Network."

Coming from virtual obscurity to worldwide fame is something many up and coming actresses dream of.

And although Rooney is the first to admit how lucky she is, she also wary, saying: "My fear with a movie like this is the kind of exposure you get from it. I think that can be death to an actor"