The Exorcism of Emily Rose

Based on true events, The Exorcism of Emily Rose tells the story of a 19 year old girl (Anneliese Michel in real life) who died as a result of an exorcism at the hands of the family's priest. The marketing studios has made this look like a horror, but it's not, it is actually a courtroom drama headed by the priest's lawyer who defends her client. Not against Emily's family though, as they wanted to get Emily exorcised, but against the State, and the Devil himself.

Laura Linney stars as Erin Bruner, a high profile lawyer fighting for senior partnership at her firm if she gets her client to plead guilty and go to jail for a while. But Father Moore (played by Tom Wilkinson) doesn't want to. All he wants is to go to trial so he can have the chance to tell Emily's story, even if that means that his sentence could be longer.
Erin, while not believing that demons played a part in all this, agrees to the trial, getting in trouble with her bosses, and the demons too, who start playing with her too.

Linney gives a great performance here. She's so good as a lawyer, very believable at it. See the magnificent Primal Fear for more proof.
Tom Wilkinson is also great, specially in the exorcism scenes. And finally Emily who is played by Jennifer Carpenter. She is so freaky looking. All trashed down and possessed. And the scenes between her and Wilkinson exchanging words in Aramaic were excellent.

While the movie is not a horror movie, there are really great scares, and the fact that the source of fear is not tangible but spiritual made it even more frightening and unnerving.
Great work by writer and director Scott Derrickson creating a very creepy atmosphere, specially during the exorcism scenes which are just brilliant.
Add the wonderful performances and The Exorcism of Emily Rose is an excellent movie that really scares, and scares hard.