Reese Witherspoon in Wild (Anne Marie Fox/Fox Searchlight Pictures )

What propels a young woman in her thirties, a hiking and outdoors novice, to tread 1,100 miles on the Pacific Crest Trail, from Mexico to Canada, on a punishing, penance-seeking undertaking? And in 1995, meaning no GPS, cell phone connection, only a map and compass? That’s just one of many questions regarding the behavior of Cheryl Strayed.

In the opening moment of this existential angst-in-the-wilderness drama, a bruised and limping Strayed (Reese Witherspoon) sits wearily on a pile of rocks, takes off her right hiking boot and sock, and examines the bloody pulp of her big toe. With one deep breath, she grabs the toenail and rips it off. Then she loses her boot as it tumbles down the mountain slope. Within the first two minutes, she understandably lets out a piercing scream. No commentary is necessary.

It’s observational and visceral moments such as this that make up Wild’s most appealing moments, based on Strayed’s 2012 memoir of the same name. They even manage to steal viewers’ attention away from the amazing vistas—like Oregon’s Crater Lake—and make Strayed’s mutterings and on-the-trail monologues superfluous. It’s often obvious what she’s feeling and thinking anyway, and so the extra dialogue makes this loner, one of the few women traveling the trail solo, seem like a chatterbox. On the first day of her trek, shortly after she hits the trail in the Mohave Desert, for example, she muses, “Oh my God. What have I done?” And she reassures herself, “You can quit anytime.” With such an expressive and instinctual actress at director Jean-Marc Vallée’s disposal, one wishes that he had trusted the audience, and his actress, a bit more; Witherspoon’s never been accused of being too enigmatic. (One other concession to conventional storytelling is that the star never wears a hat or gets sunburned, though she journeys without shade for weeks.)

Anchoring the film almost entirely, Witherspoon feels liberated from the constraints of the burdensome title of “America’s Sweetheart,” in a role that gives her the opportunity to get down and dirty. What might have been forgotten over the years is that she has consistently made off-center choices, a fact that has been overshadowed by her Legally Blonde stardom. Back in 1998, she played Susan Sarandon’s rebellious (and also topless) daughter in the neo-noir Twilight. Recently, she has ventured far and away from sunnier roles to more complicated, darker turns (Mud). She’s well supported by a cast that features actors normally not seen outside of an Alexander Payne film, playing hikers or locals that Strayed meets en route. Speaking of Payne, Witherspoon’s appearance in his satire Election had a bite to it, though not as sharp as Strayed’s.

Part of Strayed’s rationale for this months-long, physically grueling endeavor stems from her vow, heard in a voiceover as she writes in her journal, to “walk my way back to the woman I was,” to the person her mother knew, before she deliberately sabotaged her marriage by pursuing back alley trysts and snorting—and then shooting—heroin. Though Laura Dern is nearly a decade older than Witherspoon, she’s ideally cast as Strayed’s deliberately optimistic single mother, Bobbi. (Cheryl prefers Emily Dickinson, Walt Whitman; mom loves James A. Michener.) Strayed has the most patient and supportive husband-soon-to-be-an-ex in the form of Thomas Sadoski, who saves her just in the nick of time from a bad druggie boyfriend and who’s an ideal candidate for a support group for jilted good husbands, along with Luke Wilson from The Skeleton Twins.

Snippets from the likes of trip-hoppers Portishead, Leonard Cohen, Simon and Garfunkel, and Bruce Springsteen serve as musical transitions and segues to flashbacks to Cheryl’s past, helping to shed some light on her need for atonement. Through the elastic editing, Wild becomes untamed, surreal, and a jumble of memories that are connected, though not clearly at first.

Fortunately, there are only a few scenes that feel shoehorned in to explain her actions. The most strained occurs when an opportunistic journalist from the Hobo Times pulls up in his convertible where she’s hitchhiking and interrogates her, making the point—in case we missed it—that most women “can’t walk out of their lives.”

Sure, the screenplay may over-explain the reasons why Strayed went from college student and wife to promiscuous heroin user (and it conveniently skips over how she kicked the habit). At its best, though, Wild lets you into her interior, sometimes spectral world, offering a nonlinear roadmap of her life. It’s as though David Lynch made a narrative film by way of Gloria Steinem.

The film holds the audience’s interest from the get-go. This cipher of a character goes her own way: She doesn’t aim to please, which makes her all the more intriguing. She remains thorny and still mysterious, despite the sometimes heavy load of exposition.