1: Light Fan
This person is as close to normal as a fan can get. They like the book/movie/actor, but it doesn't take over their whole lives. You can have a pleasant conversation with them, and they don't try to force their fandom down your throat.
2: Medium Fan
There are 2 ways this could go-they could talk about the thing they like a lot, but still retain some concept of normal conversation, or they could be outwardly normal, pretending they don't like it, then go home and write NC-17 fan fiction until the wee hours of the night.
3: Hardcore Fan
The people you rarely see, because they are watching/reading/fapping to their obsession 23 hours per day. When you talk to them, they speak of nothing but that and how awesome it is, and you are afraid to disagree because they will probably claw your eyes out.