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Just because Reese Witherspoon is getting Oscar buzz for her portrayal of a heroin addict who tries turning her life around by hiking the 1,100-mile Pacific Crest Trail in Wild, that doesn't mean she was always so sure about starring in the project.

In the film, based on Cheryl Strayed's best-selling memoir of the same name, the Oscar winner not only appears shooting up drugs, but there are explicit sex scenes that sometimes include nudity.

"I was scared before I even started the film, so I did all my sort of emotional preparation before I started," Witherspoon told me last night at the movie's premiere at The Academy in Beverly Hills. "I remember sitting down with Cheryl before I started the movie and going, 'I don't think I can do this. I'm too scared.' She just held my hand and said, 'You're not only going to do it, you're going to be great and I believe in you. That meant everything to me."

Reese Witherspoon, Wild

Fox Searchlight Pictures

Director Jean-Marc Vallée recently told me he was impressed by Witherspoon's dedication. "It's beautiful to see her let go and get out of her comfort zone and say, 'I am going to produce it, star in it and I am going to do it in a way that's going to be tough and I'm going to be naked and raw, both physically and emotionally,'" he said.

Witherspoon didn't wear makeup for most of the movie. "She could have told me, 'I'm 36, don't show me this way, but she said, 'Shoot! Shoot whatever you see,'" Vallée said. "She arrived on set and it was like, 'Ready, Reese?' and she said yes and took the shirt off and put the needle [in her arm] and then in her ankle."

What a year it's been for Ms. Witherspoon. She also produced Gone Girl.

"It's been so exciting to see the conversation changing about women in film and strong female leads in film, not just the afterthought of fourth lead, but actually as interesting dynamic characters," she said. "Whether you like them, don't like them, at least they're starting a conversation."

Wild in in theaters on Dec. 5.

Check out more from the premiere tonight on E! News at 7 p.m. and 11 p.m.