I am a player asking this question for my GM.
So please put any story things in a spoiler and I'll have him come read it.
We are currently in Broken Moon and he said either in this book or the next (he wasn't real clear) possession of some of the PC's could come up again. He is not sure how to handle it. None of us felt like we did a very good job with it last time it came up.
He thinks there are rules for it in Council of Theives, but we don't have that.
Can the possessing spirit make the body do absolutely anything? Does extremely out of character actions cause a new save?
Line of sight? Line of effect?
Through walls? Through doors?
What save? What can affect the save beside protection from evil?
Can the possessed travel from the site of possession?
How can the party get rid of the possessing spirit?
Can the possessing spirit be damaged without hurting the possessed?
Channel positive energy, cures, ghost touch, ethereal, control undead, holy water, holy weapons, etc... What works or doesn't against a possessing spirit?
I know it seems like a lot of questions. But he said he couldn't find any rules on how to do it or how to end it.
Thanks for any help you can give!
As far as I recall, there are no generic rules for possession. The only rules that exist are the ones in the stats for the possessing monster. The details depend completely on what sort of creature is doing the possession. In the case of Broken Moon, the stats for the creature are included in the book.
I don't think the stat block in the book answers all of the questions that you pose, but anything that isn't answered in there is something that the GM will just have to decide for himself.
The possession in question gives very specific rules because it is the exact same thing as a spell in the core book (trying not to give away the encounter here). The spell is referenced in the monster description, and he should check that spell for any limitations on possession (offhand I don't believe there are any). Additionally, the monster description already gives ~very~ explicit rules for the effects of positive energy affecting both the possessing spirit and the creature being possessed, so he needs to re-read those.
Things like line of effect are covered as well, as are the saving throws and the like. The possessed can travel as far as it likes, and it won't break the possession. Getting rid of the possessing spirit is covered within the monster description. All in all, it's a nasty little encounter, but depends wholly on which PC ends up getting possessed (if any). In the game I ran, it was a gnome caster that got posessed and so the encounter was laughably easy. If it had been the orc barbarian? That would have been scary.