Reese Witherspoon had panic attacks "for weeks" before filming Wild.
The 45-year-old actress admitted making the 2014 movie - which is based on Cheryl Strayed's memoir about getting her life back together by hiking the Pacific Crest Trail - was the biggest career challenge she's ever faced and she turned to hypnosis to calm her fears before she started work on the project.
Speaking to Tracee Ellis Ross for Interview magazine, she admitted: "I was so scared to do that... I had hypnosis, I was so scared. I was having panic attacks for three weeks before I started.
"There was the nudity, sexuality, and drug-use aspect, but also being alone on camera with no other actors. I hadn't ever been alone in scenes for days and days. There were probably 25 days of the shoot where I had no other actor opposite me. It was just me and a camera and a backpack. I was like, 'Is this going to be so boring?'"
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And the Big Little Lies star admitted making the film changed her forever after pushing her to her limits.
She said: "I don't know if I'll ever work that hard again, but it changed me on a cellular level."
But Witherspoon - who has Ava, 21, and Deacon, 17, with ex-husband Ryan Phillippe, as well as Tennessee, 8, with spouse Jim Toth - thought Strayed's book was "beautiful and sacred" because it had taught her a lot.
She said: "[It] was so beautiful and sacred to me because it spoke to me so deeply about how we as women have to save ourselves. There's no mother or father coming to save us. There's no spouse.
"I thought it was radical that at the end of the film, she ends up with no family, no money, no job, no partner, and she's happy."