"The imperative was to create interesting roles for women," she says.
"I've never thought I was right for every single role, and I'm actually thrilled to help craft opportunities for actresses."
In fact, she says she's known Pike for years and was excited to see her play someone so different.
As a producer, Witherspoon added that she was just there to support and learn all she could from Fincher, the guy who helmed films such as Fight Club, Se7en and The Social Network.
"Mostly, I was thrilled one of our first movies was going to be directed by one of the great American filmmakers of our time," she says.
Witherspoon herself is garnering acclaim for her turn in the upcoming movie Wild, another adaptation she's producing.
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Her production company also recently co-opted the rights to an Australian author's novel - Big Little Lies by Liane Moriarty - along with Nicole Kidman's production company.
The pair are set to star in the limited TV series adaptation (similar to the length of True Detective), which is being written by Emmy winner David E Kelley.
- WENN