Last night I re-watched House of the Devil, and while it certainly didn't scare me like the first time I saw it (what horror film does) I still hold that it's probably the best horror movie of the 21st century. I recalled and noticed a few new things that help to make it so:
The movie runs for twenty-five minutes before Samantha arrives at the house. This is vital to the movie's success: We get a full picture of Samantha's life, her situation, and her financial dilemma. Essentially, we get a character who is fleshed out and who we care about, which thus makes us more frightened when she starts to get frightened. This is the same move that Psycho makes. It hardly seems a coincidence that both of these films begin essentially as urban dramas with sympathetic female characters who then create a horror story out of their lives due to the pressing need for cash.
Another nod to Psycho seems to be the fact that there's this old woman living upstairs that we seem to hear but do not actually see until the end. The genius of House of the Devil is that it's set up as a babysitter thriller and is that, except that breaks new ground by harkening back to Psycho. Is that a contradiction? It's more like a paradox: using tradition to subvert a different tradition is a thing of beauty when done well. Here it certainly is.
Finally, note that that Mr. Ulman is Samantha's last pursuer during the film's thrilling and shocking climax. Tradition would state that the last is the worst, the scariest, the hardest to kill. Yet West surprises us by making Mr. Ulman, who had previously been one of the spookiest parts of the movie, more vulnerable than terrifying. It's a brilliant move by West because it makes us feel assured that Samantha will be able to escape (and again we really care because she's been developed as such a strong, likable character), which then makes us all the more stunned (spoiler) when she takes the gun to her head.