Throughout the game you'll have to combine the usage of both human and demonic powers. Each level is designed to have challenges that pretain to the current state of your body. For instance, at the beginning of the game, you'd be able to reach through flames and other extremely harmful things, allowing you to grab things, flip switches, etc. Later, when half of you're body (and head) is demonic, you can switch between demonic and human thought, allowing for all sorts of interesting puzzles.
Uhm sorry, but are you making this game or me?
No offense, input is appreciated but the way you wrote that kinda sounds like you are thinking for me.
God I love that movie.
EDIT: wow this is already getting loads of attention, I better start spriting soon.