"There were small descriptions in the script, but when he started describing what we were going to do, that's when I started to panic," she reveals. "That's, like, three per cent of the movie, but it took up a tremendous amount of fear in my mind because it's daunting ... Sometimes I was just terrified. Like a cat on a raft... 'You can't make me do it.'"
"I would have fired myself a couple of times during rehearsals because I was so scared, oh my God," she adds. "I got my s*it together, but it took me a while."
But though filming was admittedly a grueling experience for Reese, she is proud to take on such a "complex" role.
"It wasn't as if there was a lack of roles being offered to me," Reese says bluntly about fighting for the role of Chery Strayed. "It was the dynamic aspect of playing a really interesting, complicated person that was not readily available. Honestly, I don’t know a woman who isn't complicated. It's strange that you don't see many complicated women on film; complicated meaning complex, I should say.'"
This notion of being "America's Sweetheart" obviously bothers the Southern star, who also dishes to Glamour magazine in another new interview that she doesn't identify with the title.
"I certainly didn't self-identify as anyone's sweetheart," she says. "I'm friendly, but I don't think I'm sweet. I'm honest."
But taking on one of her most challenging roles to date and being happily married with three kids, Reese is certainly in a great place in life, which she shares comes with age.
"I don't think I realized [in my twenties] that no one else makes you whole," she says. "You have to take responsibility for your own happiness. That took me until I was about 31 to know ... No relationship, no children, no nothing is going to make you a happy person."
That being said, she couldn't be happier with her husband, CAA agent Jim Toth, who she's been married to since March 2011. She was previously married to actor Ryan Phillippe, who she divorced in 2006 after nine years together.
"Well, I have to say a very healing part of my life is my wonderful husband, Jim," she gushes. "It's really wonderful being with somebody who supports me, who loves me, who is my number-one fan. That is such an incredible experience. You have to be ready for those things. I had to be in a place where I felt all those things about myself."