My office-mate Joe noted something the other day --
In Twin Peaks Leland and Agent Cooper are possessed by Bob, the spirit of evil.
Starbuck dies and returns as an Angel for some mysterious god, probably good (though they leave open the question as to whether this is some kind of Gnostic god).
Locke died, apparently for real if the actor is to be believed (or if he even knows), and is then he is possessed by (let's call him) Easu.
Interestingly Quantum Leap turns out to show this theme from the other perspective.
Why are we so interested in our human characters being possessed by, or swapped out for, cosmic forces?