From PEOPLE Magazine Click to enlarge
The hair

Mara's first big step toward transforming into the new movie's androgynous hacker heroine: chopping 12 inches off her brown mane and dyeing it black. "It was a surreal day," says the 26-year-old actress, until now best known as the sweet-faced preppie who dumped Jesse Eisenberg in The Social Network. "But I was already getting into character. I was ready!"

The piercings

She added six piercings-in her ears, her eyebrow and one, well, R-rated location. (Her nose and lip rings are fake.) Did they hurt? "It was fine," Mara says. "I wasn't freaked out by the hair-cutting, the piercings, the dyeing. The bleaching of my eyebrows was the most startling thing."

The training

To measure up to Swedish author Stieg Larsson's avenger, Mara kickboxed, did push-ups until she could clap her hands between each one and "skateboarded to get that 14-year-old-boy stance," she says.

The mind-set

Raised in suburban Bedford, N.Y., Mara is football royalty: Her great-grandfathers founded the Steelers and the Giants (her big sister is actress Kate Mara, 28). Still, "Salander didn't seem that foreign to me," she says. "Rooney was able to draw on being shy and introverted like Lisbeth," says producer Cean Chaffin. "But she's got a good family. Lisbeth is alone." For Mara, "I found it quite easy to slip into Lisbeth's skin. It was harder to come out."