In many ways this has been the most anticipated casting announcement since Scarlett O'Hara landed Gone with the Wind. Why? Swedish author Stieg Larsson's Millennium Trilogy are bestsellers all over the world (more than 40 million copies sold in 44 countries) and dominate the current NYT bestseller list; the three Swedish Millennium films have scored in multiple countries as well. The first two are doing modest art-house business in the U.S.
Word was the studio resisted going with an unknown--Mara was the only American among the finalists (Emily Browning, Sophie Lowe, Sarah Snook and Lea Seydoux). But clearly, Fincher had his way. He hired an actress who he trusted. And those of us who haven't seen her work will discover her when we watch The Social Network, which opens the New York Film Festival September 24. Sony has set a December 21, 2011 release date for Dragon Tattoo.
In short: A star is born.