A few things have led me to consider the notion of a Possessor Demon that is possessing the Sorcerer that Bound it. In Sorcerers Soul it talks about the possibility of a complicit possession, where the possessed person is working with the Demon, and may even switch off control of the body with the Demon, but this raises some questions.

I guess the primary one would be, how long does such a switch take? Specifically, would it be able to happen as one of the "half" actions during a combat round for a Demon with Fast? This would allow the Demon and Sorcerer to both act during the round (if the Demon is willing of course), but could allow for some interesting scenes as the Demon and Sorcerer switch off, both taking swings at the enemies (I'm picturing a berserker here).

The other question I guess is how well does that work out in a game? The sorcerer being possessed by his own Possessor Demon that is. I realize it would take a lot of control away from the player during times when the Demon is active, but are there other issues I'm not seeing just yet? Or does this fall into more of an "advanced" playstyle similar to having the players play the Demons does?

The "bit extra" of Fast I've never allowed to be a whole extra die worthy action.  I've only ever allowed it to justify actions that would otherwise seem slightly implausible given the fictional circumstances.

In fact, hosting possessor demons in the Sorcerer is quite nasty for situations that require precision timing because the wrestling for control itself eats up an action.  Generally at the top of something like a fast past action sequence the Sorcerer/Demon have to decide whose going to be in control.  If one or the other doesn't like how things are going he has to seize control as an action just like ordering the demon normally.  This is subject to all the usual abort/defend etc rules of a complex conflict.

I've had players go for the whole "possessed by the spirit of an animal god" motif and be EXTREMELY upset by the results in actual play.

Jesse

So let's say the Demon is in control, and is trying to kill the opponent, and the Sorcerer decides to take control at this point, would it play out as follows?

Demon rolls stamina to attack Enemy.
Sorcerer rolls will to take control.
Demon has a choice to just roll one die of defense, and potentially complete his action anyway, or cancel his action and go to full defense.

Yes.

The one die option is especially risky because if it fails, the demon will lose his action anyway since the Sorcerer is back in control.  Depending on the fictional circumstance (like a game focusing on body horror) I MIGHT allow the demon's action to go off but the defender would get all of the Sorcerer's victories as bonus dice.  Sort of a last second "twitch" response.

Jesse