The Oscar winner is starring as Cheryl Strayed in the film adaptation of the 2012 best-seller Wild: From Lost to Found on the Pacific Crest Trail, about her 1,100-mile trek.
Wearing a heavy backpack, Witherspoon, 37, who also produced the film, has re-created Strayed's journey of self-discovery through freezing snow and blistering heat.
The film's director Jean-Marc Vallee told Entertainment Weekly that his leading lady "went method with the hair and no makeup" for the film. "We wanted to see Reese out there, not vain at all, not concerned with her image, and she went for it," he added. "Wait until you see her in the film. It's a new Reese."
Strayed visited the set of the film in October and wrote in a Facebook post how "extraordinary" it was to see Witherspoon "dressed in the clothes I wore on the trail, her hair the same style and color and cut as mine then, with my beloved/loathed Monster on her back packed just the way I packed it."
Wild hits theaters Dec. 5.