In her latest film Wild, Reese Witherspoon tells the true story of Cheryl Strayed, a woman who lost her way after her mother's death and numbed the pain through promiscuity and drugs, until she sought to find her path again by walking 1,100 miles of America's West Coast on the Pacific Crest Trail.

Witherspoon, the larger than life petite blonde who came to prominence as Hollywood's southern belle, starring in films like Election, 2001's Legally Blonde and Sweet Home Alabama, believes her fans won't be shocked by seeing her in sex and drug-taking scenes.

"I think my audience has grown up too, and the women who saw me in Election or Legally Blonde aren't 20 years old any more, they're 35. They have kids and real-life experiences," says the 38-year-old, who was born in New Orleans and raised in Nashville, Tennessee.

"I've evolved as a human being, and also as an actor. You take chances, and you never know whether audiences are going to accept you or laugh in your face. I've been really touched that people have been so receptive to me doing something different."

The role has already seen her pick up a Golden Globe nomination (though she lost out to Julianne Moore at the weekend) as well as a Bafta nod, but the actress - who won an Oscar in 2006 for her performance as June Carter in Walk The Line, the film based on the life of country music legend Johnny Cash - admits she recently had to have a conversation with her 15-year-old daughter, Ava, about the more controversial scenes.

"I had to explain it to her, because people are starting to talk," says Witherspoon, who also has son Deacon, 11, from her previous marriage to actor Ryan Phillippe, and two-year-old Tennessee with her second husband, talent agent Jim Toth.

"I said, 'Ava, I've got to tell you something... I'm naked in the movie!'" reveals Witherspoon, laughing. "She was like, 'Mum that's so weird!' And I said, 'I had to be brave, and couldn't just tell the parts I was comfortable with, because Cheryl was brave enough to tell her whole story, so I had to tell the parts that I was scared to do'. But she's very proud of me."

Strayed turned her experience of tackling the unforgiving three-month trek into a bestselling memoir, which was published in 2012. Nick Hornby, who adapted the book for the screen, has since said that her whole life story, from her difficult childhood to the fallout following her mother's death, would make a compelling movie in itself.

But it's Strayed's determination to turn her life around that most touched Witherspoon when she read an early manuscript.

"It was one of the most profound books I'd ever read in dealing with loss and grief, and the idea that no one's coming to save you in your life, you have to save yourself," explains the actress, who was arrested in 2013 for disorderly conduct when she and Toth were pulled over by a police officer (s he later admitted she'd had "one drink too many").

As soon as she'd finished the book, Witherspoon - who'd recently launched the production company Pacific Standard with producer Bruna Papandrea - called Strayed with the hope of making a movie about her story.

"I found her to be every bit the spiritual and emotional person that you'd expect. She's no nonsense, cuts through all the 'BS' and just tells it like it is - the same things people really responded to in her book." Herself included.

There have been times "when I laid on my kitchen floor and cried a lot", Witherspoon confesses.

"And then I think you get to a certain point and go, 'I don't want to feel like this any more', and you figure your own s**t out. No one's going to figure it out for you. Feeling weak is human, and you can't apologise for being human. We all feel weak sometimes, and sad and distraught and lost."

She insists she didn't embark on the project with the intention of playing the lead role - "but then Cheryl said, 'I really want you to play me'".

"I didn't know if I was capable of it," the actress admits. "I was terrified of this movie before I started. I tried everything to get out of it but Jean-Marc [Vallee, the director, who also helmed last year's Oscar-winning Dallas Buyers Club] and Bruno [Papandrea] wouldn't let me."

Although she considers herself 'outdoorsy', nothing could prepare her for the physical challenges of the shoot.

"There was climbing up the side of a mountain and balancing on river crossings, and marching through chest-deep snow and falling into a freezing river... I had no idea it was going to be as hard as it truly was.

"But the whole point is, Cheryl didn't know what she was doing, so Jean-Marc really didn't want me to be prepared, or go hiking or be in good shape," she adds. "And he never let me see the tent beforehand. I couldn't work it out. He must've filmed me for two or three hours, so when you see me swearing, that was for real!"

Then there was the "monster" rucksack that Strayed, in her naivety, had set out with.

"The boots and backpack became a part of me," Witherspoon notes. "Sometimes the prop master would have to say, 'You can take it off'. But as Cheryl says in her book, there's something amazing about realising that everything you really need in life, you can carry on your back. It is so liberating, a beautiful idea."

The star, who will soon be seen in the crime-thriller Inherent Vice with Joaquin Phoenix, and The Good Lie, about a woman who helps Sudanese refugees, hails Strayed as a true inspiration.

"She could have made other choices. She could have become a drug addict, gone down that rabbit hole and never returned. But she decides not to. She pulls herself out, and that is incredibly inspiring for anyone who is struggling with their life in any kind of way.

"So many people feel they are alone and have no one to help them," Witherspoon adds. "This story gives voice to the idea that you can save yourself, and that's a really powerful thing."

EXTRA TIME - LONE EXPLORERS

:: The Conquest Of The Pole (1912) - This silent French film follows the misadventures of a comical group of explorers on an expedition to the North Pole. Initially a box office flop, it's now hailed as a masterpiece.

:: Jeremiah Johnson (1972) - Robert Redford stars as an ex soldier searching for a life of solitude.

:: Into The Wild (2007) - Sean Penn wrote and directed this adaptation about one man's travels across America and life in the Alaskan wilderness.

:: 127 Hours (2010) - Danny Boyle directed James Franco in this movie, based on true events, about a climber forced to hack his own arm off to free himself after falling and getting trapped in a canyon.

:: Tracks (2013) - Mia Wasikowska plays a young woman who spends nine months crossing the Australian outback.

:: Wild is released on Friday, January 16