So I figured out how angelic possession in SPN works. It’s simple and elegant, really. As a caveat I am not a physicist so I could well be talking out my ass here. Just bear with me.
The problem with possession is that it seems to defy the principle that no two objects can occupy the same space at the same time. There are exceptions to this principle and the solution here is that angels aren’t technically objects.
We think of ourselves as solids but really that’s only true to a certain degree. On a sub-cellular level – molecular and below – we’re a collection of particles moving together in sync. There are of course gaps in between, sort of like the inverse of a froth. We exist on four dimensions: three planes of physical space and a tangent point in time.
Angels, on the other hand, are comprised of wavelengths of pure energy without mass, likely without particles (and if they are particulate I’d imagine it’s still massless particles, eg photons), that simultaneously exist on multiple planes of existence – more than four dimensions (see 6.03). Since we’re talking metaphysics here it’s pretty clear that they extend beyond the known electromagnetic spectrum and may also be composed of an unknown parallel spectrum of energy. This explains a freaking lot about them.
It explains why the human mind isn’t really equipped to conceive of their true form: they exist in a state of being unlike that of any documented life form and extend well beyond the visible spectrum. It explains why looking at a “naked” angel burns out the eyes: part of their radiation is harmful, to say nothing of the sheer intensity. It explains how they read minds, hijack TVs, and listen in on police radio: that’s “all waves”, as Castiel pointed out in 8.08. It explains how they make themselves invisible: a simple matter of withdrawing from the visible light spectrum, which for an angel must be something like a human sitting on her own feet.
But the most relevant to this great heap of blather is possession. The massless, non-particulate nature of angels means that they can occupy the spaces in between the particles of a solid living being (read: human) without disrupting those particles – if the frequencies at which the human’s particles vibrate and those of the angel don’t clash. This explains why only certain humans are fit to be vessels. The more powerful the angel, I’d imagine, the more precise the match has to be. If angel and vessel aren’t properly matched the former ends up bashing up against the latter and causes the body to start unraveling or combusting from the atomic (or even subatomic) level up, as seen with Lucifer’s possession of Nick.
The fact that an envesseled angel is a conscious collection of energy waves occupying the interstitial space of a human’s atoms gives it the ability to manipulate the body’s particles: moving through the dimensions on which an angel naturally exists (flight [assumed by humans to be teleportation], time travel, alternate reality travel), healing, imperviousness, superhuman strength & reflexes, etc.
Now there’s actually a third element at play here: the human soul. As quarterclever points out here, we’ve yet to see an angel take possession of a soulless body (braindead, corpse, etc); there’s enough proof (Lucifer in 5.03, Michael saving Dean in 5.13 – though really he was saving Mary to make sure the boys would be born) to state that the body has to be alive, at least, and able to give definite (if not necessarily well-informed) consent, though it’s still unclear as to whether SPN’s mythology considers this consent to be an action of the human’s consciousness as an extension of the physical self or of the soul.
In any case, I posit that souls occupy a spectrum of energy parallel to the electromagnetic one (occupied at other “frequencies” by corrupted forms of the human soul such as demons, ghosts, etc, and likely also parallel to angels’ Grace), so that’s a third set of vibrations that have to match that of the angel – an extra level of compatibility necessary for possession. In SPN, human souls are potent power sources, so the probable trade-off here is that the soul acts as both a buffer between the physical body and angel’s potentially-deleterious radiation (as previously discussed), and as a battery of sorts for the angel itself.
Now if all this is true, why don’t demons and ghosts have to worry about compatibility?
That’s a pretty simple answer too: they were once human. In fact they are literally just human souls gone wrong. They’re smaller than angels and ostensibly inhabit the same range of planes as living people while on Earth. All told they’re far more compatible with the human body already – far less alien than angels who were never human to begin with. This means they fit in much more easily; but it also means they’re not capable of the same level of sub-cellular manipulation.
How do Grace and the Fall work, then?
I don’t bloody know. ¯\(°_o)/¯ Actually I do have a vague notion but I’m a little fried and still have just under two hours of workday left so it’ll have to wait. If and when I figure it out fully I’ll post about it.