As far as Rooney Mara, Fincher has said he was looking for an actress that could scare him. You can see in this movie why she could — what did you see in her?
EISENBERG: She’s incredible, and she’s very difficult to impress — and I spent a few days trying to! That’s part of what made her perfect for Erica, as well. She’s sitting with the smartest guy in the world and she doesn’t appreciate what’s sitting across from her.
SORKIN: I was around when David fought for her for The Girl With the Dragon Tattoo, which they’re shooting right now — that’s why David’s not here. Every actress that would come to mind wanted the part, people with higher profiles than Rooney has right now — though Rooney’s profile is about to change considerably. But David knew what he wanted, and damn the torpedoes.
Source: Rolling Stone