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, Grave Halloween (2013) 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.
In order to differentiate the twin sisters, Natalie Dormer had to dye her hair black while playing Jess and keep blonde hair as Sara. This was also to insert ying-yang symbolism; Sara was always the more innocent and pure sister, hence her blonde hair, while Jess was always more psychologically troubled and sad, hence her black hair.
According to the Spanish comic author Juan Torres, the plot, title, and look of the film are very similar to "El bosque de los suicidas," a Spanish comic book by Torres and Gabriel Hernández that was published in 2011.
The trivia item below may give away important plot points.
Not counting the yurei or corpses in the forest (including Hoshiko who was revealed to be a yurei), the body count is 4: Sara and Jessica's parents, Aiden, and Sara Price. Michi's fate is unknown as the film ends with Sara's disembodied ghost attacking him.