this quote about cordyceps from this article in a scientific magazine is basically how i've always imagined possession to work in horror stories. the demon/entity/whatever doesn't alter your perception, displace you, or control your mind, they just take over all your bodily processes, bypassing your brain entirely. they force your mouth and vocal cords to form the words they want you to say, force your limbs to move in the direction they want you to go, and force your lungs to keep expanding and contracting and your heart to keep beating, even through excruciating pain and horrific injury, so you can't even self-sabotage and your friends and loved ones are discouraged from trying to stop you for fear of what harm they might cause you to do to yourself to escape. you're a passenger in the driver's seat of your own car, and the hands on the wheel, though outwardly apparently the same pair you've always had, are no longer your own.