Of course, this is not the first time the 38-year-old has been feted by the critics. Almost a decade ago, in 2006, she won an Oscar, a Bafta and a Golden Globe for her measured, heartfelt performance as June Carter Cash in Walk the Line.

comedies

It was the culmination of a successful early career that had begun when, at 14, Witherspoon made her astonishingly assured debut in the family drama The Man in the Moon. She had an amped-up charm that translated just as well to comedies such as Alexander Payne's 1999 high-school satire Election (her portrayal of the nightmarishly ambitious student Tracy Flick was nominated for a Golden Globe) and the pastel-hued Legally Blonde, the 2001 film that brought her international stardom. These successes were interspersed with adaptations such as American Psycho and Vanity Fair, but for the most part the films with which she remains associated in the public's mind are her rom-coms.

When I interviewed her in 2010, she may have been glass-half-full to a fault, her impeccable Southern manners dialled high, but she was also thoughtful and obviously intelligent; a complex person with darkness as well as light.

But in the years that followed her Oscar victory, Witherspoon made a series of films that invoked at best shrugs, and at worst sneers. It wasn't catastrophic - in fact, the Oscar had helped her to become the best-paid actress in Hollywood, able to command $15m-$20m-a-film - but her work didn't live up to her own hopes. Like many actresses, she found that Hollywood wasn't interested in making films led by complex female characters, and particularly not those in their mid-30s.

By 2010, Witherspoon was becoming increasingly frustrated. She was tired of the romcoms, but when she and her agent looked for more demanding roles, they came up blank. "I think literally one studio had a project for a female lead over 30," she recalled recently. "And I thought to myself, 'I've got to get busy'."

The situation was all the more frustrating given that the studios appeared to have been working on false assumptions: that female-led films only interested female audiences, and that romantic comedies were safe bets.

Women have opened some of the biggest films of the past 12 months - think of Jennifer Lawrence in The Hunger Games: Mockingjay - Part 1, or Angelina Jolie in Maleficent - proving that having a woman in the lead role is no bar to success.

In 2010, Witherspoon decided to take matters into her own hands, joining the Australian producer Bruna Papandrea to set up their own production company, Pacific Standard, with the goal of making the type of complex, female-driven stories that were largely absent from the screen.

In Wild and Gone Girl, they found characters with the depth they had been looking for. In fact, if it weren't for Witherspoon, Wild might never have made it to the big screen: she spotted the book before it was published, and snapped up the rights within days of reading it.

injected

A memoir of the 1,100-mile hike that helped Strayed get over the death of her mother, it tells how in her grief she slept with strangers in alleys and injected heroin.

"My agent told me, 'The only way this is ever going to get made is if an actress attaches herself to it because she wants to play that role'," says Strayed. "So you need not just an actress who passionately wants to make it, but one who's powerful enough in Hollywood that she can get it done. Reese was the first person we sent it to. She read it immediately and called me. And that was it."

Witherspoon moved just as decisively with Gone Girl, proving that she has a keen eye for a potential hit.

Witherspoon's bookish side was something neither Papandrea nor Nick Hornby, who wrote the screenplay for Wild, had counted on. Hornby recalls their first meeting at a party in 2010, where they had what he describes as "quite a surprising talk about books. She started talking about a short story of mine that had been in an anthology about 10 years ago," he says.

"You can see in her performances that she's super-smart, but her literary side was a revelation to me."

Papandrea also had her expectations turned on their head. "Would she really read everything, take the calls and move as quickly as you need to? That was a real test for me," she says, recalling her initial concerns. "I was kind of amazed: she's a force of nature."

positive

As well as producing, Witherspoon also began taking small acting roles in striking films, such as Mud, in which she appeared as the object of Matthew McConaughey's affections. The film went on to score 98pc positive reviews according to Rotten Tomatoes, the highest rating of any film on Witherspoon's CV. She has taken another small role in Paul Thomas Anderson's Inherent Vice (due out later this month), and in the summer she'll also be seen in Atom Egoyan's Devil's Knot.

Perhaps because they'd handed over control of Gone Girl to Fincher, who brought his own production team with him, when it came to Wild, Witherspoon and Papandrea decided to stay close to the project.

They kept the film to themselves until they had a finished script, and shot it on a small budget. Witherspoon threw herself into what she has since described as the hardest role of her career. The backpack that the actress hauls around on screen was as heavy as the one Strayed had carried herself, and the director, Jean-Marc Vallee, banned her from wearing any make-up (not a minor consideration for a major female film star). She has already been nominated for a Golden Globe and a Bafta with an Oscar nomination sure to follow on January 15.

In November, news leaked of Big Little Lies, a television series based on a novel which Witherspoon is developing with Nicole Kidman and David E Kelley, creator of Ally McBeal. She's also developing two fantasy franchises. The message could hardly be clearer: you underestimate Witherspoon at your peril.

Wild is released on January 16