My title did mean to say "Spirit Possession and Deity Assumption" but I ran out of room. Also this is the beginning of what I want to post. This section is extramely huge so I am going to have to do it a little bit at a time.
What is possession- It's having control of something or ownership.
How does possession work- First things first unless the person is dead, we must remember there is a soul inside the body, so for possession to work the soul that owns the body needs to move over and give room for the deity/spirit to move in. Gods are a lot bigger then we are,so only a small part of them when being possessed will actually enter us,no matter how much we could try and squish ourselves. The rest of Gods being will either be swirling around the body or not even enter the space at all. Though its said that the more "humanlike" the deity is the smaller they are in comparison to the older and less "human" gods.
Some people are what you would call " wired" psychically( possibly neurologically) for being a horse (the term horse is actually borrowed from the Afro-Caribbean religious traditions), while others are not. This is not saying that people who are not natural horses will never experience possession, it's just they have a very slim chance. Having a gift like this is not something someone can really practice and master, the possession really depends on the Gods and spirits themselves, if they don't want to possess you, they won't possess you.
There are many different reason why a God/Spirit would want to horse someone a few but definitely not all of the reasons would be for information purposes.This would fall into place if a God would like to talk to a person "face to face" rather then getting their information translated though a spirit-worker and having the chance of the information being messed up.Another reason would be for training and by this I mean sometimes things can simply not be taught to you through words,images etc.It may require a God/wight to enter your body(does not have to be fully)and "motor you through" this.( This is how Raven Kaldera got taught pathwalking with the help of Loki) As well sometimes the beings may want to use a body to teach someone else.Other reasons for this may be the purpose of public devotion.This is a real valuable public service because for some people seeing someone horse a deity and then maybe getting the chance to talk to them face to face is a very important thing for them, as well as for the Gods themselves.
There are different levels of deity assumption that a person can experience,starting from the lowest level to full-on possession.( This list started to circulate a long time ago around the neo-pagan community and was said to be attributed by Willow Polson)
Enhancement- This level is when a person is speaking about a deity.This is sometimes done though third person invocation,story telling etc. Doing this will help enhance the person/people's understanding about the deity,make a connection to that deity or feeling the presences of that deity. Enhancement is based more on the knowledge about that deity rather then having a strong in-depth relationship to them.
Inspiration- This level is similar to the enhancement process where you are still telling the story or giving a invocation but the difference is your speaking from the perspective of the particular deity. Instead of you simply just speaking about them you are actually speaking for them. Instead of stating that this deity did this and this,you start to state this deity wants this and this. You start to speak from the deities view point. The person who is doing this should not only have a understanding about the deity but also have a good connection with them as well and have a understanding of what they want.
Shadowing- This is where the deity "rides along in your head", shadows your mind. This is where the horse can feel the deity like they are right behind them,speaking to them clearly.(some can even keep up running conversations in shadowing) This is not full-on possession because the horse still has complete control of their body and their actions.Now you are opening yourself up to that deity so you should make sure that you have experience with this deity as well as a strong relationship.
Integration- This level takes you a little bit further into things where now your actually speaking like "you are" the deity.(Speaking in first person) This is normally only done at the high point of a ritual.(It would be similar to the Wiccan's "drawing down the moon")You become like a vessel repeating the exact words for the deity,the words that you speak just flow out of your mouth and you don't control what you say or how it is said.
Aspecting- The line between Integration and Aspecting is very slight.This is where you become a full channel for the deity's energy,however you still have control of your body because the deity has not fully taking it over yet. This can be referred to as co-consciousness which means that the deity's energy and mind as well as your own energy and mind both are shared and exist together. This is less dangerous,difficult, and much easier on your body and soul then full-on possession and also this can be mistaken for full-on possession if you have never been ridden before.This would be a stage that most spirit workers go into to channel deceased loved one,minor spirits,etc.
Possession- This level is the most strongest and intimate out of all the above. This is where the deity actually comes into the body of the person and places their soul,energy,and personality fully into that body taking full control. The deity can then freely move around in that body and talk directly to the audiences.Usually the horse feels very distant from the whole thing(like seeing and hearing through the view of being very far away or like they are underwater)Some will experience almost or complete unconsciousness of the whole thing where they will only remember bits and pieces of the event missing huge chunks in between or have totally memory loss. This can be related to the situation of a deity driving a car and your in the back seat behind glass or your locked in the trunk.
Hope to get more up on this topic soon :D
Sources: Drawing down the spirits- The traditions and techniques of spirit possession by Raven Kaldera and Kenaz Filan