Behind the Mask is like two movies. The first half of it is a mockumentary, with a journalist following a man who aspires to be a serial killer like Jason, Michael Myers, and Freddy Krueger (who exist in the world of the movie as real killers, not movie icons). This half has a good bit of comedy and pokes fun at slasher films, with the namesake of the film, Leslie, explaining how he carefully scoped out a girl to be his "survivor" and set up things in and around the house (where the killings will take place) to ensure that no one gets alive. It suggests that the actions of the supernatural killers are planned ahead with great thought and not done spur the moment. It is a business, and it takes a lot of work. This is very enjoyable, especially for those who have seen a lot of slasher movies.
The second half, however, is like a regular movie. A regular slasher, at that. There are one or two surprises, but they're very predictable and not surprising at all. Nothing great happens, and there isn't even any gore to spice things up. People get hit quickly and die quickly. This half is boring and should have been left on the cutting room floor. Sure, the movie wouldn't have made a lot of sense, but it would have been a better movie. The second half is exactly what was being made fun of in the first half, a generic slasher. It's a shame.
I want to recommend this movie, but I also want to say that you should fast-forward when the camera crew puts down their cameras and things get real. That way, you'll get to see what happens, and you won't have to waste forty minutes watching the same drivel you've already seen in a million other movies.