Read the W article
that The Playlist linked. It goes into more detail about Mara's auditioning and transformation to Lisbeth Salandar:
Mara didn’t blink. “David added the rape scene at the last minute, and I said, ‘Ohmigod! They must be really serious.’ They did one test, then another a week later. They shot me in the subway in L.A. in full hair and makeup with a motorcycle. Every day they had a new request. On a Monday morning, David called me in, and I said, ‘What do you want me to do to my hair now?’ I was at the end of my rope. He told me I had the part. I hadn’t even read the script yet.”
...Five days later Mara moved to Stockholm. She began training—learning to ride a motorcycle and kickboxing. The (temporary) dragon tattoo proved to be tricky: Fincher did not want it to look Asian or like it came out of a comic book. He finally settled on a dragon that could have been drawn by Escher—more like an engraving and quite beautiful. In one “very intense” day, Mara’s eyebrows were bleached, her hair chopped, and her lip, brow, nose, and nipple pierced. “I didn’t even have pierced ears,” Mara said, still sounding a little shocked. “They put four holes in each ear, and, weirdly, that hurt the most. It was all very organized. With David, everything is measured and carefully considered. He wants what he sees in his head.”
...“I wanted her from the beginning,” Fincher stated. “Rooney may be a trust-fund baby from football royalty, but she’s levelheaded and hardworking. It’s so odd how who people are comes out in auditions. We didn’t make it easy for Rooney, and there was no way to dissuade her.”
It also talks about how this version will be different from the book and previous film, and why Fincher was finally interested:
The script, which captures the novel’s bleak tone (its original Swedish title was Men Who Hate Women), was written by Academy Award winner Steven Zaillian, who wrote Schindler’s List, and it departs rather dramatically from the book. Blomkvist is less promiscuous, Salander is more aggressive, and, most notably, the ending—the resolution of the drama—has been completely changed. This may be sacrilege to some, but Zaillian has improved on Larsson—the script’s ending is more interesting.
...Aside from the visceral and cinematic nature of the material, Fincher was also intrigued by the villains in The Girl With the Dragon Tattoo. They were not politicians or dictators—instead, the top bad guys are big businessmen. “Fascism has worked its way out of politics,” Fincher said, “and gone into high finance. Today Woodward and Bernstein would be investigating corruption in the financial arena. I was interested in that. And, of course, the girl.”
Most amusingly, he thinks The Social Network
"...on Social Network, I didn’t really agree with the critics’ praise. It interested me that Social Network was about friendships that dissolved through this thing that promised friendships, but I didn’t think we were ripping the lid off anything. The movie is true to a time and a kind of person, but I was never trying to turn a mirror on a generation.”
...By his reckoning, Fight Club and, especially, Zodiac (neither of which were box office successes) are films, while The Social Network (which is a box office smash—close to $100 million in America alone) is simply a movie.
“It’s a little glib to be a film,” Fincher maintained. “Let’s hope we strove to get at something interesting, but Social Network is not earth-shattering. Zodiac was about murders that changed America. After the Zodiac killings in California, the Summer of Love was over. Suddenly, there was no more weed or pussy. People were hog-tied and died. No one died during the creation of Facebook. By my estimation, the person who made out the worst in the creation of Facebook still made more than 30 million dollars. And no one was killed.”