"They lose their physical bodies and go into the person. They get control of the person, but not access to their memories, so while some can be good actors it gets a bit hinky for long periods of time. Only way I know to get them out is to get an angel, or wait for 'em to get out on their own. Might be possible to threaten them out, but I don't think you want to get mixed up in stuff that can scare a demon out. I dunno of a way to keep 'em from getting in, maybe load yourself up on tons of luck charms and hope for the best."