The thing i find interesting about the Regan is that i actually care about the character. I say this because is so many exorcism film since the exorcist, the character that become possessed - i just haven't really felt that i cared enough about that. And that obviously ruins the film. This is also why, i think, i dislike so many of the post-exorcist exorcism films.

Dean, that is a great point. Father Karras may be the Protagonist in the sense that the story is told through his perspective, plus he's the character into whose inner life we most enter -- his religious doubts, relationship with his mother, science vs. faith -- but The Exorcist does an excellent job spending time with Regan and her mother, enabling the viewer to connect with them as characters. I can't recall what minute it is in the movie where Regan appears with her mother's guests downstairs and urinates on the rug in front of them -- the first sign of her possession -- but I would be surprised if it's before minute 25, so again the story spends time with Regan to get to know her as a child, relationship with her mother, difficulties on the home front because of strained relations between her mother and father, etc. This issue of what is sometimes called "audience identification," referring to the dynamic whereby we connect with key characters in a story, is enormously important, and as you point out done well in The Exorcist.

I'm with you on that. We get to know that Regan is not some rebellious, Satan-worshipping, potty mouth crazy kid. She is (or was) a happy-go-lucky, humorous daughter with a sisterly relationship with her mom. Blatty and Friedkin were super smart to create a clear distinction between Regan and Regan/Demon - makes her possession feel even more unfortunate and tragic.

One of the most frightening aspects of The Exorcist derives from a choice the filmmakers made which was not to provide any definitive explanation about why Regan became possessed in the first place. It's precisely because no rationale is offered that drives home a point: This could happen to any of us at any time. It's an unspoken subtext of the story which plays upon the utterly uncertain and random nature of the universe which when we stop to think about -- something we generally avoid doing -- is truly frightening.