The music. Wendy Carlos's synth themes are frightening enough, but when the Penderecki stuff comes on, with those chanting ritualistic choruses and the jangling, discordant instruments, my bowels loosen and my blood runs cold. Kubrick's customary impersonal sterility adds a lot as well, but it's the music that kills me. The piece called "Utrenja" is the most disturbing music I have ever heard in my life, and "The Awakening of Jacob" runs a close second. I wasn't one tenth as scared reading the novel as I was watching the movie, and usually books scare me far more than movies.