This question was raised by another thread. Posting it here so I don't confuse the rules discussion going on there.
Sorcerer Steve is fighting against Possessor Pat. Steve decides that for his action he's going to attempt a quick Contain on his body, while Pat has decided to attempt to possess Steve.
If Pat succeeds in Possessing Steve, does Steve lose his action on the Contain, or can he still finish the Contain, thus (temporarily) trapping Pat there?
My guess is that after the possession, Steve can't do nothing, he's reduced to "a flickering bit of consciousness", at most he can try to stop the demon from doing something with a will roll. (this, if you don't change the way possession work in your saga from the default way in the core book)
Even if the Demon, for some reason, would like to finish the ritual, it could not: a demon can't do sorcery rituals.
Moreno's got it. Once the demon's in, the action he had in mind is annulled - much as if you were going to hit someone, but just before that, you get hit and end up with so many penalties that you are "stunned, no dignity." The intended attack simply evaporates.