The genre of the movie "Behind The Mask: The Rise Of Leslie Vernon" is horror. More specifically, it is a 2006 American mockumentary black comedy slasher that examines the conventions of classic horror movies from inside the story.
The film follows Leslie Vernon, played by Nathan Baesel, as he prepares to become a masked killer. He invites documentary filmmaker Taylor Gentry and her crew to record his planning, training and carefully designed murder spree. The documentary format lets Leslie explain familiar slasher rules, including the importance of a final survivor and the advantages of a remote setting.
The movie begins as a satirical faux documentary but shifts toward a more conventional horror presentation when the violence begins. That mixture is why it is sometimes catalogued under several labels, including comedy, mystery, thriller and slasher. Horror remains the broad, canonical genre, while “mockumentary” describes its format and “slasher” identifies its specific horror subgenre.