There's a definite risk with making a movie that has a slight case of multiple-personality disorder itself. It's a mystery and a crime story and a thriller and a horror flick all in one, and it switches between these different language codes in a way that could have worked, could have been clever. The performances are strong throughout, Julianne Moore delivers, as does Jeffrey DeMunn as her father and Jonathan Rhys Meyers in the various incarnations of whoever is in David's body at the moment. The problem is more that this devolves into a fairly trite horror movie, nothing particularly original about it, and the twist at the very end isn't surprising, or even mildly upsetting, at least not to this viewer who saw it coming a mile away.
It fails at going into some of the basic archetypal fears that could have made it truly frightening, like the inherent instability of the human psyche, something we rely no being more writ in stone than it really is, and opts instead for a vague kind of religious gloss of the old fire-and-brimstone variety, which, again, would have been fine, if it had any kind of lead-in other than the gruesome deaths of the people occupying the preacher's body.
Even the Witch Of The Hills is an archetype that could have been unsettling, but here she isn't even set up in opposition with the basic Christian morality she is supposed to act in contrast to. Instead it all comes down to having faith in a standard issue Christian God, especially when the chips are down, because if you don't a rogue damned and hill-witch cursed preacher is going to come and kill you. See what I mean? It doesn't make sense and it doesn't keep any of the promises it made in the opening. This movie is quite simply not as clever as it would like to be. I wouldn't waste my time with this one.