Okay, time to explain the cosmology the previous bit of dialog was alluding to. Again, these posts are successors (ha, ha) to my earlier post on the workings of souls; this time, we're going to delve into the exact in-fluff mechanics of spirits and possession.
Spirits, you see, are just the catch-all term for a soul (or soul-like entity) that's not occupying its natural-born body (if it ever had one- there are numerous theories on such topics). As explained before, this means an existence which transcends the physical; neither body nor brain, but the intangible essence that provides a being with free will (and through that, the potential for any sort of development. Your consciousness is the sum of your physical brain and your soul). Some common examples of spiritual phenomena are:
Possession: Possession is what happens when a spirit attempts to displace or suppress someone's soul in order to take their place. Accomplishing such a thing hinges on a pure battle of will, one where the victim not hindered by reduced mental functionality but will be highly vulnerable if they've lost their resolve. Long-term possession is only feasible by displacing the soul rather than suppressing it- a suppressed soul will retake its form the second the possessor's force of will is weak enough, but a displaced spirit can be banished and otherwise checked in its efforts to retake its rightful place. Meanwhile, one without a soul is helpless against possession and can only drive out a spirit through outside intervention.
Astral projection, transcending the physical, "like I was watching myself through someone else's eyes": Put simply, an out-of-body experience is what occurs when your soul is dislocated from your own body. Some can accomplish this at will through practiced meditation or natural aptitude; it can also occur by accident when one attempts to exert their will while the mind is in a muddled state (usually due to drugs, sleep, or intense emotion). The experience has much in common with dreaming- your lucidity will vary, as will the degree to which you can recall what transpires during the experience.
Ghosts, "spirits of the dead": It is theorized that all ghosts are astral forms, usually deteriorated, whose physical bodies expired while they were dislocated- usually the type of dislocation brought on by intense emotions, which tend to be fairly negative (though there are also ghosts who, for example, were unknowingly poisoned at a feast and are still drunkenly festive.)
Poltergeists, "haunting": When a spirit's actions have an obvious intent and intellect behind them- written messages, the use of trickery, and so on- then the spirit is in a lucid state, and to a medium will likely appear as they did in life (albeit stylized to match their personal perceptions). Poltergeists are the opposite end of the spectrum, incoherent and largely formless; they take confused actions based on mixtures of emotion and can perceive little about the material world beyond that which their attention happens to fixate upon. Poltergeists are often associated with evil spirits, but what few realize is that such poltergeists are often not the act of the spirit itself- rather, it is the displaced soul of the victim lashing out in its confusion, while their body and brain feign innocence while working towards more sinister ends. Seasoned exorcists know better than to turn their back on a supposed victim of a poltergeist.
Avatars, "he fought like a man possessed": Not all interactions between a spirit and a host are parasitic in nature. A spirit can bring a measure of power with it, especially the older and stronger ones that haven't turned down one of several malevolent paths and thus stripped away their distinct traits to become an established type of ethereal predator. Instead, these spirits become something higher, more abstract- they take on the characteristics of an archetype, but they also dictate those characteristics. A spirit of vengeance can concern itself with justice or self-gratifying retribution. A spirit of fire can seek to warm or consume. (There are many more details to get into here- at least a full post's worth. In fact, in this process lies the key to how a man becomes a god. But for now, we return to the topic at hand.)
Such distinct spirits can and do form functional relationships with hosts, where the native soul suppresses the outside spirit the majority of the time but the two will switch places during certain conditions. Of course, the soul is not something that can just be swapped out like an article of clothing. Even if the mind is willing, an incompatible soul and native spirit will be like a knight astride a wild horse, always seeking to cast off whomever's playing the role of the rider. Instead, a spirit that would have a more harmonious relationship seeks a soul whose nature and cause match its own. Rarely is this a conscious arrangement; for example, there has been many a fearsome berserker who described their memories of battle as though it were a dream, until they came back unto themselves surrounded by bodies with a bloody weapon in hand. The rider bows down to the horse, and the horse in turn becomes the rider.
Hope this has all been thought provoking. Next up: What I actually had in mind with that alternate plot for Ghost Rider. Something more permanent that possession happens when a spirit comes into a body that lacks a soul. . .
Today's image is by "Su", from pixiv.