"I'm naked quite a lot in the movie, and I thought, she has it in the book, and she should have it [in the movie]," the Social Network star, 26, tells the January issue of Allure. (Costarring Daniel Craig, Dragon Tattoo is based on late Swedish author Stieg Larsson's worldwide best-selling crime thriller, the first in the Millennium trilogy.)
"Because of all the tattoos and the makeup and the piercings, and the physical transformations my body has to go through, it would always feel sort of like I was in costume, even if I was naked," she told Allure. "It just felt like a good one to get--a necessary one to get."
Indeed, Mara -- who beat out a bevy of A-list actresses for the part, including Scarlett Johansson -- didn't sweat the graphic nudity or the drastic changes the part demanded.
"Nudity is such a huge part of the character in the book, so I never thought twice about it," she says. "There was no time for hesitation. I didn't have time to second-guess anything or be scared; I just showed up and was comfortable."
Also not traumatizing: Lopping off her long locks to become Salander, who uses her hacking skills to help author-turned-investigator Mikael Blomkvist (Craig) solve a decades-old mystery and uncover a vast conspiracy.