The 38-year-old Witherspoon has been one of Hollywood's highest-paid and most bankable box office draws since her breakout starring turn in the 2001 comedy Legally Blonde.
But she failed to gain credibility as a serious actress until her 2006 Oscar win for her role as singer June Carter Cash - the wife of superstar Johnny Cash - in James Mangold's biopic Walk the Line.
Wild is beautifully shot by Vallee, whose previous feature Dallas Buyers Club began its journey to Oscar success in Toronto last year.
Reese Witherspoon in Wild. Photo / AP
Devastated by the death of her mother in 1991, Strayed self-destructs - destroying her marriage, becoming hooked on heroin and even becoming pregnant by a stranger.
In a bid to find her way out of despair, she embarks on a 1,770 kilometre hike along the US Pacific Crest Trail.
The film - with a screenplay by Nick Hornby (High Fidelity, About a Boy and An Education) - shows Witherspoon in explicit sex scenes, injecting heroin and badly bruised from lugging a 75-pound "monster" backpack through the awesome California and Oregon wilderness.
The actor said she expected during filming to have "newspapers stuffed in the pack," but Vallee insisted that "it actually has to be heavy."
"It changed the way I walked," Witherspoon said. It "dug into my shoulders, and my body quickly got tired of carrying this thing.
"And then after five and a half weeks of carrying it on my back, I got so used to it. It was like an appendage, and I missed it when it was gone," she said.
During the film shoot, Vallee also ordered the actors not to apply any makeup, going as far as covering the mirrors in their on-set trailers to prevent lapses.
"It was raw," said Witherspoon, but "if Cheryl could be brave enough to tell every part of her story, I had to be brave enough to throw away my vanity and go for it."
Reese Witherspoon in The Good Lie. Photo / AP
In The Good Lie, directed by Philippe Falardeau, Witherspoon plays a secondary role to the actors playing the refugees, several of whom actually lived the experience depicted onscreen.
Emmanuel Jal and Ger Duany were born in South Sudan and were among the more than 20,000 Lost Boys of Sudan who were displaced - and some orphaned - during the 1983-2005 civil war.
- AFP