Rooney Mara: The girl with the very bright future

It was one of Hollywood's best-watched talent searches for years. And it ended with the sort of unlikely twist that has entranced 40 million readers and turned The Girl With The Dragon Tattoo into the most valuable piece of literary real estate since they sold off film rights to Dan Brown's back catalogue.

Rooney Mara, a virtually unknown actress whose profile is so low that even her age is being hotly debated (she is, depending on who you believe, either 24 or 25), has been chosen to play Lisbeth Salander in the US film versions of Swedish author Stieg Larsson's Millennium Trilogy of thriller novels.

In landing top billing in the film, which will open next Christmas, Mara beat off competition from an extraordinary array of stars. Everyone from Scarlett Johansson and Natalie Portman to Carey Mulligan and Emma Watson has featured on the list of names being tipped for the job. Mia Wasikowska, the protagonist in Tim Burton's recent Alice in Wonderland, was among the final half-dozen contenders, though she did not sign a test-screening deal. Four finalists are understood to have spent recent weeks in auditions with the film's director, David Fincher, who will begin shooting on location in Sweden next month.

Mara's appointment was announced via a press release issued on the Facebook page of Sony Pictures on Monday afternoon, which summed up her entire life story in a "biography" which ran to just 10 words. "ROONEY MARA will appear this fall in David Fincher's The Social Network," it read.

In fact, the actress has quietly been considered a rising (if low-profile) figure in Hollywood circles for most of the past year. She appeared in the modish Michael Cera film Youth in Revolt, and also in this year's remake of A Nightmare on Elm Street. Her turn in the forthcoming Fincher film – a biopic about the founders of Facebook – will hit cinemas in October. Mara's deal with Sony includes an option for two sequels, The Girl Who Played With Fire and The Girl Who Kicked The Hornets' Nest.

Daniel Craig will play the film's male protagonist Mikael Blomkvist, and Robin Wright will co-star. But the complex nature of Mara's role is considered to be the most pivotal to the eventual success of the film. Lisbeth is a bisexual computer hacker who suffers from Asperger's syndrome, is accused of murder, and has a sexual relationship with the much older Blomkvist. "Lisbeth Salander, Stieg Larsson's fierce pixie of a heroine, is one of the most original characters in a thriller to come along in a while," added Sony's release. "[She is] a gamin Audrey Hepburn lookalike but with tattoos and piercings, the take-no-prisoners attitude of Lara Croft and the cool, unsentimental intellect of Mr Spock."

The film-makers were essentially presented with a blank canvas when casting the role. Thanks in part to the fact that Larsson died before his bestselling books were even published, all we know is that Salander is relatively short and is aged 24 but can sometimes look as young as 14. Mara, who is believed to measure 5ft 3in, certainly ticks those boxes. Her casting nonetheless represents part of an expensive gamble. Although the huge popularity of The Girl With the Dragon Tattoo, which has sold 10 million copies in the US alone, ought to guarantee buzz for the picture, that is by no means guaranteed to help it turn a profit.

Fincher was responsible for the acclaimed films Fight Club and The Curious Case of Benjamin Button. However both failed to translate critical plaudits into blockbuster status. The dark nature of The Girl With the Dragon Tattoo may further dent its appeal to the masses. A final difficulty he may have to overcome involves the fact that fans have already been able to watch Swedish-language versions of the Millennium Trilogy. Those movies have already made more than $200m (£130m) worldwide.

The competition

Scarlett Johansson ScaJo was reported on Monday to be a "shoo-in" for the role. But the New York Daily News is now eating its words.

Carey Mulligan In April, reported The Sunday Times, the British Oscar nominee was on the verge of a deal to play Salander. "Hollywood insiders say Mulligan can expect to be paid up to £10m for the three films," it claimed.

Emma Watson After the Daily Express claimed she cut her hair to win over casting agents, the Harry Potter star ordered her agent to deny she would be auditioning. The paper's report was "absolutely false", she said.

Mia Wasikowska One of the few rumoured contenders actually in the running, the Australian star of Tim Burton's Alice in Wonderland reportedly pulled out, citing other commitments.