Witherspoon, 38, agreed to film some naked scenes for the big-screen adaptation, reported E!Online.
"I had to make a choice when I decided to do the film on how much I wanted to bear. I have a real story to tell here because Cheryl was completely honest when she wrote her book.
"It (naked scene) was scary and challenging. It's not just a movie about somebody walking. It's really about someone who feels profoundly hurt and lost," she said.
The mother-of-three is getting all kinds of Oscar buzz for her role.