The tall doctor at the hospital, the one Hallie saw in the very dark corridor is the 6' 8½" tall actor Javier Botet, who also portrayed the Slender Man.

Sony and Screen Gems became extremely nervous about the film during post-production due to ongoing court cases, negative publicity surrounding the Creepypasta legend, and extremely negative responses to early test screenings. The film was released with very little fanfare, and was not screened for critics.

The father of the victim who was almost killed by two girls that worshipped the Slender Man spoke out against the film. He said he felt disgraced by Hollywood making a film about events that led to tragedy.

Hallie and Lizzie's last name is the same as the real last name of the original creator of Slenderman, Eric Knudsen, who goes by the pen name Eric Knudsen.

The Slender Man first appeared on the "Something Awful" forums in a series of photographs edited to depict a tall, humanoid entity unnoticed by other people in the image but almost always surrounded by, or in close proximity to, children. Since then, many have speculated inspiration and origin of Slender Man in popular media, folk tales and ghost stories from hundreds of years ago, the earliest being a reference to Der Großmann in a German folk story written in 1702. But actually there is no German folk story that's called Der Großmann (which would translate to The Tall Man).