Rooney Mara "resisted" acting because she was worried she wouldn't be as good as her sister.
Although she took drama classes when she was younger, Rooney, 30, admitted she didn't take it seriously during her teens because Kate, 32, was so good.
She explained: "My sister started doing musical theatre, and I'm not very musical - that was never an option for me. So I just took some drama classes and loved it. For someone who was quite shy as a child and is more of a guarded person, it was a nice way to release now and then. It was something I wanted to do but I resisted it for most of my adolescence.
"I thought Kate was so brilliant and I didn't want to do it and not be good. But I also sort of knew from a very young age that I didn't want to be a child actor, that I wanted to go to college, then start acting when I could be taken seriously."
And since making up her mind to act, Rooney has been careful about what roles she has chosen as she doesn't want to be embarrassed by her work.
She told The Sunday Times Culture magazine: "I made a decision, if I'm not going to be able to do the things that are great and inspire me, then I'm not gonna do it at all. I was scared but I was wilful about it. I'd had a few experiences where I was embarrassed by the work I'd done and I thought, 'Well it's not worth it to me.'"