Why THE EXORCIST is scary: “That Thing Upstairs Isn’t My Daughter” Scene
I don’t have the answer to the general question: What makes scary? Obviously that is a subjective judgment, and THE EXORCIST is as good an example as any of what can scare some folks badly and make others laugh in derision.
This movie terrified me as a teenager, and I saw it at home on a television (although alone, which was probably not a good idea; I still turn the channel quickly if I come across it). All the demonic stunts, and in particular the voiceover by Mercedes McCambridge, were tremendously effective and are what come to mind first and foremost.
But apart from the showy stuff, I think this scene with Ellen Burstyn very nicely sums up what makes this movie scary.