Natalie Dormer actually went to the Suicide Forest with her Japanese driver for research. She ventured five meters off the path to take photos and her Japanese driver would not step half an inch over the path.
Aokigahara, known as the Suicide Forest or Sea of Trees, is a 35-square-kilometre forest lying at the north-west base of Mount Fuji in Japan. The forest has an association with demons in Japanese mythology, and is a common suicide site. A sign at the start of the main trail urges suicidal visitors to contact a suicide prevention association. There have been two recent movies inspired by this premise, Shawn4Ever (2012) and The Sea of Trees (2015), both released before The Forest.
When Natalie Dormer's character is first browsing information about the Aokigahara forest, the photos on her screen are real, taken from actual recovery parties that, annually, scan the forest to retrieve human remains. Among the pictures (though blurred) is the infamous "half face" man that also inspired the film's poster.
The poster for the film features Natalie Dormer's face with the bottom half removed, and the dividing line forming a silhouette of treetops and dangling nooses. This bears a clear resemblance to an infamous image of the body of an actual suicide victim of the forest, a bald man found in such a decomposed state that his jaw had fallen off and strips of flesh were hanging from where it had been. The image is also replicated in the film, albeit in a slightly less grotesque form.
The name of the Japanese guide, Michi, fittingly translates as "path" in Japanese.