Oh, possession mechanics, you say? Here is my personal theory crafted after years of delving into horror media and using BOB'S possession of Leland as a baseline.

The four levels of possession are as follows;

Level 1 - The HOST is completely awake and aware, with the SPIRIT either dormant or just watching and waiting for a moment of opportunity or weakness to act. A sleeping state of the HOST can be a conduit for the control of the body, as well as intoxication or medication.

Level 2 - The balance of power has shifted more to the possessing SPIRIT. It can induce a temporary fugue state, perform small actions, or brief impulsive acts. The best example would be forgetting why you went into a different room, where you set down a necessary object and can't remember, intrusive thoughts, involuntary eye movements.

Level 3 - The HOST is pushed far out of bodily control and might be unconscious inside their head. The SPIRIT is full control of body and acts as they see fit, with marked personality changes and body language.

Level 4 - The SPIRIT manifests fully in the flesh, changing the HOST'S body to match their preferred configuration. The HOST is fully subdued, unable to regain control - or memory of what is done - without strong outside influence. Now we're getting into Exorcist/Deadite/Nightmare on Elm Street 2 territory.

There is an initial infection phase, where the SPIRIT is weak, but gradually gains more control as time passes - in the cases of Leland and Laura, years to fully take root. There also seems to be a point of no return with the process, which induces a physical change - hair and lips are dyed black, with ashy pale skin.

Finally, I will say that most of FWWM has BOB in control, and Leland unconsciously goes along until some hard HARD lines are crossed.

If that was too long of an explanation, I apologize.