I finally had the opportunity and inclination to watch The Exorcism of Emily Rose, which to my surprise is an excellent movie because it is so perfectly done in every aspect [at least from my amateur perspective] and so believable. I am also wiling to accept that it was an accurate account of actual events, but I contend that either the actual events or the movie was at some level intended to deceive us about the nature of the typical persistent forms of Satanic possession, or even to convince us that possession is a myth because so few of us have any first-hand evidence of the phenomenon depicted in the movie. (This is what I describe as "relegating it to the Twilight Zone.") Just ask yourself how a form of possession such as the one depicted in the movie would serve Satan's purposes. I'd say not very well.
Actual possession for the most part is typically very subtle and far more destructive. Satan's overarching goal is to serve the Devil's agenda of driving the human spirit out of the world (while Lucifer strives to lure it out by tempting us to escape into a fantasy-world), and this is best done by manipulating mankind into self-destructive behavior on a large scale, such as by becoming a globally acknowledged "expert" on "global warming" and telling gargantuan lies, knowing that they could kill billions of people. Then there are apparently forms of possession which produce something like a Ted Bundy who is driven to prey on humans; but then Bundy was also active in politics, indicating a possible mass-murder agenda in addition to literal hands-on serial murdering. Someone suffering from a form of possession resembling that depicted in the movie is hardly likely to do much more than get locked up or killed.
Then there's the matter of Satan's intense desire to conceal the sorts of activity (SRA) which are intended to lead to Satanic possession, and which I assume are effective in many cases because of the evidence that such activity has continued over the course of hundreds of years. It is possible that Emily Rose became possessed spontaneously due to a wide array of factors, but based on my reading of the works of Rudolf Steiner, this is a rare occurrence. (If it were frequent, then there would be no need for the soul-trap known as Satanism.) It is also possible that the particular entity which had taken possession purposely drove her to behave in a manner intended to deceive us about the nature of possession.
So, at some level, the movie was in my opinion intended to distract us from the true nature of the forms of possession which serve Satan's agenda, and how they develop. It is possible that God, in this case through St. Mary, used this deception as claimed in the movie: to serve as evidence that the spiritual world exists and influences events in the physical world.
If you're in the mood for a truly chilling and enigmatic movie, you can't do much better than this one.