So, at night in Sonic Unleashed there are these people with little purple ghosts coming out of them. These people are really depressed. It turns out they are that way because Evil Spirits, and part of the game involves magically curing them.
This would make sense, if not for the fact that at the start of the game a mad scientist's superweapon cracks the planet apart.
I think it's the people who aren't hanging their heads in despair who are possessed.