The Exorcist is almost 40 years old. It speaks to the power of that film that demon-possession copycats are churned out year after year in an attempt to cash in on its still-considerable pull. The Conjuring is the latest to follow in its rather sizable footsteps, and it captures some of the classic's appeal.

The key to making this sort of horror film effective is obvious: take the ostensibly least threatening character and put the gnashing, hate-spewing demon inside his or her body. The wolf in sheep's clothing here is the drowsy Lili Taylor who plays the matriarch of the Perron clan, a family of seven (Taylor, her husband played by Ron Livingston and five daughters) tormented by a particularly malicious spirit after they move in to a new home in Rhode Island.

The emergence of said spirit is fun stuff, if horror is your thing. It starts out almost playful -- grabbing a leg of one of the daughters as she sleeps or slamming a door, only gradually becoming more bold.

What isn't so welcome is the parallel storyline featuring Vera Farmiga and Patrick Wilson as Lorraine and Ed Warren, paranormal investigators and semi-famous authors who decide to come to the rescue of the Perrons as their circumstances become more and more dire.

Every possession flick needs someone to try and exorcise the (quite literal) demon, but Farmiga and Wilson aren't entertaining bit players. They are the main characters, an odd plot choice that undermines the degree to which you ought to pull for the Perrons to be extricated from their predicament.

The annoyance goes beyond that if, like me, the first thing you think of when you hear the word "medium" is a particularly hilarious South Park episode. Maybe this is something that's very particular to me, but I resent hucksters who pretend that they can talk to spirits, duping unsuspecting simpletons in the process. So, yeah, in that sense making them the central heroes of a horror film is going to get deep under my skin. But I contend that it's disruptive even if that isn't the case.

For most of us, horror films are really just scary fantasy scary films. Making a couple of snake oil salesmen the main characters in this sort of world shatters that fantasy, and, with it, spoils the more promising parts of The Conjuring.