Yes, even though the movie and book was based on a true story, it also made to be more rich in the fiction department than the truth department.

Now though I never saw the movie or even read the book, I have found the true story behind it all online. I thought I'd post the link for you to read it, so you can see that people can be possessed by demons.

You can read the story here.

I'll also post a two more links that also go hand in hand with possession.

World Famous Exorcist Talks About Harry Potter Luring Kids Into The Occult

Exorcism-The Facts