That I am regularly hauled out of my burrow every Halloween like some furless and demonic “Punxsatawney Phil” always brings a rueful smile of bemusement to my lips as I lower my gaze and shake my head, for the humiliating God’s-honest truth of the matter is that while I was working on "The Exorcist," what I thought I was writing was a novel of faith in the popular dress of a thrilling and suspenseful detective story – in other words, a sermon that no one could possibly sleep through -- and to this day I haven’t the faintest recollection of any intention to frighten the reader, which many will take, I suppose, as an admission of failure on an almost stupefying, scale.

But it’s true!

Before "The Exorcist," I was a comic novelist ("Which Way to Mecca, Jack?") and writer of motion picture comedies like the Inspector Clouseau farce, "A Shot in the Dark." Who knew from scary?

When I saw the novel’s early reviews (Newsday: “One of the most terrifying stories since 'Dracula!'”), for a time I found myself prematurely in that unbalanced and “tricky age” for writers of comedy described by the great James Thurber in his rollicking "A Preface to My Life," as a period “when they take to calling their office from their home, or their home from their office, asking for themselves, and then collapsing in hard-breathing relief upon being told they ‘were not in.’”

Yes. Confusion had made its masterpiece!

Oh well, sure: every year on the date I put out the pumpkin with the cutout eyes and nose and face and the basket full of Snickers and Mars Bars beside it; but I do keep wishing – oh, ever so wistfully and – let’s face it, hopelessly – that "The Exorcist" be remembered at this time of the year for being not about shivers but rather about souls, for then it would indeed be in the real and true spirit of Halloween, which is short for the eve of All Hallows or All Saints Day.

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.

William Peter Blatty is a writer and filmmaker. "The Exorcist" is his magnum opus: he also penned the subsequent Oscar-winning screenplay. His most recent works including the novels "Elsewhere," "Dimiter," and "Crazy." October 2011 marks the 40th anniversary of the publication of "The Exorcist." For this occasion, Mr. Blatty has re-edited the book and added a new scene.For more information, visit the HarperCollins website.