Critics have been raving over Rooney Mara's transformation into goth girl Lisbeth Salander in The Girl with the Dragon Tattoo, and it appears that notoriously OCD director David Fincher did think of every last detail. No, really, every last detail:

"There was a lot of discussion that went into my merkin for the movie, because I was naked quite a lot," Rooney says in a new interview with Metro. "And I don't think a lot of people will notice all of the attention that went into that. But there was a lot of discussions around that."

See, in the book Lisbeth dies her hair black, but her natural color is strawberry blonde, so "We had a special merkin made that was, you know, strawberry-blonde so that it would fit," Rooney says.

A key detail of Lisbeth's character are her tattoos and piercings, including one in her nipple that caused quite a ruckus when it was featured on a teaser poster for The Girl with the Dragon Tattoo (left). While the tattoos are fake, Rooney did go under the gun to get real piercings for the film:

"I thought, 'She has it in the book, and she should have it [in the movie],'" she tells Allure about her new hardware. "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. It just felt like a good one to get -- a necessary one to get."

The Girl with the Dragon Tattoo opens in theaters December 20, but you can catch up with star Rooney Mara right here at MrSkin.com!