Image copyright Getty Images Image caption Witherspoon plays the lead role in Wild and also co-produced the film 
 
 Hollywood star Reese Witherspoon has said her new film Wild was the most challenging of her career to date. 
 
 The Oscar-winning actress plays a young woman who goes on a 1,100-mile hike across the Pacific Crest Trail after the break-up of her marriage. 
 
 Based on the memoir by Cheryl Strayed, Witherspoon's role involves scenes of explicit sex and drug taking. 
 
 "It's the hardest I've ever done for many different reasons," Witherspoon told reporters in London. 
 
 "The physicality was really difficult, but after that was the emotional part of it," she continued ahead of the film's European premiere. 
 
 "The sex scenes were the hardest thing for me to do. I've never had to do anything like that in my entire life. 
 
 "I had to do all the parts of the movie, the parts that made me feel uncomfortable too, because it is about emotional honesty," the 38-year-old went on. 
 
 The actress, who also co-produced the film, has been tipped as a frontrunner for next year's best actress Oscar. 
 
 She previously won the award in 2006 for her role as Johnny Cash's wife in Walk the Line. 
 
 Wild shows Witherspoon's character carrying a heavy backpack on a gruelling 94-day journey of self-discovery across hot desert and icy mountains. 
 
 The actress revealed the backpack had originally been filled with newspaper, only for director Jean-Marc Vallee to exchange it for a heavy load to make her movement look more realistic. 
 
 Image copyright TIFF Image caption The actress's performance could see her in the running for a second Oscar 
 
 Witherspoon told the BBC News website that she had benefitted from having the real Cheryl Strayed on set during filming. 
 
 "In some ways I was a little scared that she would be watching and judging me. But it would actually help me get into the scene." 
 
 Strayed said on Monday that many of the conversations she had with Witherspoon had not been about the making of the film. 
 
 "They were about our lives, our childhoods and relationships, talking about ourselves as mothers - everything from the mundane to the extraordinary," she told the BBC. 
 
 "There was some [talk] about how to hook up the backpack, but the more important work was opening ourselves up to each other." 
 
 British novelist Nick Hornby, who wrote the film's screenplay, said Wild could not be described as a so-called "chick flick". 
 
 "It's about grief and heroin addiction and promiscuity and being really tough physically and mentally," said the Fever Pitch and High Fidelity author. "So it's not really like any chick flick I've ever seen." 
 
 Wild has its European premiere at the BFI London Film Festival later and is released in the UK on 16 January. 
 
 Vallee's previous film, Dallas Buyers Club, won Matthew McConaughey and Jared Leto acting Oscars earlier this year. ||||| The movie, based on Cheryl Strayed's memoir about hiking the Pacific Crest Trail, is broken up by flashbacks too much but Witherspoon makes us feel Strayed's predicament in our bones. 
 
 “Wild” is based on Cheryl Strayed’s passionately funny-sad, Oprah-approved, bestselling 2012 memoir, but it doesn’t come across as a “safe” project. Reese Witherspoon, who plays Strayed, may be bucking for another Oscar – and just to make sure we know it, she doesn’t wear any makeup – but, as Hollywood movies go, it’s fairly uncompromising. 
 
 The movie, like the book, employs a back-and-forth flashback structure to depict Strayed’s 1,100-mile solo hike on the Pacific Crest Trail from the Mojave Desert to Washington State, which she undertook in the summer of 1995 when she was 26 and despairing from the death of her mother (well played by Laura Dern) and a broken marriage. (Strayed legally changed her surname from Nyland in 1995.) The hike began impulsively, as a way to break off bouts of sex and heroin addiction, and turned into a grueling soul-searching lasting more than three months. 
 
 Jean-Marc Vallée (“Dallas Buyers Club”), directing from a script by Nick Hornby, begins the movie right in the middle of the hike, with Strayed agonizing over a bloody toe in a too-tight boot. Her backpack, nicknamed “the Monster,” is almost as big as she is. In visual terms, Vallée is constantly pitting the diminutive Strayed against the broad expanse of the wilderness. 
 
 Danger is ever-present, not only from wild animals and rattlesnakes and parched landscapes and freak snowstorms, but also from the many men she encounters on the trail. (And the hikers are almost all men.) The sense of a single woman as prey has rarely been so chillingly conveyed in a movie (at least in a movie that wasn’t intended as a thriller). 
 
 Witherspoon uses her own undoubted discomfort with the physical demands of the role to make us feel Strayed’s predicament in our bones. (At one point our heroine slogs 20 miles to a water tank only to discover it’s empty.) The film is another in a series of memoiristic recent movies – along with “Into the Wild,” “All is Lost,” and “127 Hours” – about how confronting the wilderness outside compels a showdown with the wilderness within. What makes this one different is that Strayed doesn’t abide by the strong-and-silent ethic governing those films. The strongest emotion in the movie, her main motivating force, is the deep love she still holds for her mother. 
 
 Too much of “Wild” is broken up by flashbacks that tend to dissipate rather than enhance Strayed’s trek. At times she is swallowed up almost to the point of vanishing by the immensity of the vistas. The various encounters she has along the way with hikers, both platonic and predatory, have a schematic feel, as if we were proceeding chapter by chapter through her journey. 
 
 I’m also not quite sure I buy the spiritual uplift Strayed derives from the hike’s completion. It could just be that she is naturally high from being royally pooped. But Witherspoon accomplishes a very difficult feat: Often wordlessly, she makes us understand how Strayed could be less lonely in the wild than in a city full of people. Grade: B (Rated R for sexual content, nudity, drug use, and language.)
– In Wild, Reese Witherspoon plays a woman who embarks on a 94-day, 1,100-mile hike alone, and the actress says the role is "the hardest I've ever done"—and not just because she had to carry a heavy backpack while filming so her movements would look realistic. The movie is based on Cheryl Strayed's memoir about hiking the Pacific Crest Trail after her mother dies and her marriage ends, and the film includes explicit sex and drug scenes. "The physicality was really difficult, but after that was the emotional part of it," Witherspoon told reporters before last night's European premiere in London. The raw role could pay off, as Witherspoon's performance is generating no small amount of Oscar buzz. "The sex scenes were the hardest thing for me to do. I've never had to do anything like that in my entire life," Witherspoon continued, according to the BBC. "I had to do all the parts of the movie, the parts that made me feel uncomfortable, too, because it is about emotional honesty." And Strayed herself was there during filming, which initially "scared" Witherspoon, she said, but ultimately, "it would actually help me get into the scene." British novelist Nick Hornby, who wrote the screenplay, underscored Witherspoon's take on the movie: "It's about grief and heroin addiction and promiscuity and being really tough physically and mentally," he said. "So it's not really like any chick flick I've ever seen." (Click to read another recent Witherspoon interview in which she opens up about her other new movie.)