"I'm trapped in this body and can't...get...out..."
A person's body is taken over by some foreign intelligence. It's not brainwashing, because they still have control of their own mind—but that's all they have. Either they've been reduced to a spectator in their own body, or their soul has been booted out of their body entirely. Either way, the rest of the cast will soon find he's Not Himself.
For information on specific types of hosts, look to Body and Host.
Not to be confused with the people who steal corpses.
Please put examples on the appropriate Sub-Trope and not here.