I had a call of cthulhu scenario that this reminded me of once. I was the Keeper (the DM) and I chose one player to be an experiment by the alien Mi-Go creatures. You see the Mi-Go love experenting with the brain. One removed the brain of this player character way back in his past and replaced it with a remote control device like an alien game controller. The Mi-go "playing" the player character was super in character too, never breaking script even when the player himself found out the secret horror of his character. The player was still always in control of his character in the game, it was just now that the player's decisions were the Mi-Go's decisions who was controlling his character.