This film does not throw the horror right at you but lets the viewer feel their own level of fear. This is likely due to the film being based on a real person and real events.
You don't have to believe one way or the other regarding demons or exorcisms to enjoy this movie. Instead the movie challenges you to ask is it possible. If you think it is, then the concepts are terrifying. If you don't then sit back and enjoy a good horror flick.
The actress playing Emily Rose can do some body contortions giving her presentation a realistic edge.
The story is told through a murder trial using flashbacks. For me this presentation worked as it allowed you to think as the revelations were revealed. Once again, if this is possible than it is indeed unnerving.
Peace and hope you do not wake up at 3:00 a. m.