Happy Halloween from the writer of The Exorcist

  I made the mistake of seeing The Exorcist opening night, midnight show. I thought I had a strong enough faith and that, since I believed so strongly in God and salvation, that such a movie wouldn't faze me.
  The movie theater held a certain chill, a sense of anticipation not usually present in most movies, even opening night. I am no longer a fan of the horror genre and have since learned that sleepless nights can result from exposure to unspeakable acts.
  It should be remembered that this was the first of a genre; kids nowadays laugh at The Exorcist. They don't think it's frightening at all; their vision and consciousnesses have been so seared with violent imagery and language that this historic early horror flick is amusing to them.
  It wasn't to the people who saw it in the theater in those early days. The tension was almost palpable.
  Now Blatty reveals a secret, that he thought he was writing a book of faith, one that would bolster belief in God. From Fox News, he reveals:

When I first heard, in 1949, of an actual case of demonic possession and an exorcism going on nearby while I was a junior at Georgetown University, I remember thinking, “Someday, somebody’s got to write about this, because if an investigation were to prove that possession is real, what a help it would be to the struggling faith of possibly millions, for if there were demons, I reasoned, then why not angels? Why not God?" 

While there was one particularly striking and constantly repeated paranormal phenomenon in the 1949 case, plus a letter from the exorcist in the case assuring me that the case “was the real thing; I had no doubt about it then and I have no doubt about it now,” I in fact did not base my novel on the 1949 case, but rather what my research made clear; namely, that in every period of recorded history, and in every culture and part of the world, there have been consistent accounts of possession and its symptoms going all the way back to ancient Egyptian chronicles, and where there is that much smoke, my reason told me, there is probably fire – and a lot of it, if you get my meaning. Do you? My faith is strong.

  Here's the background for the film at Wikipedia, and here and here is background for the real case.
  Here's the movie in 30 seconds done by bunnies.
  And, if you're brave, some "real" cases are here.