An example would be, say, a person claiming to actually believe they are a dalek and mean it. I have no choice but to believe they are full of shit and don't believe it and just want attention. Do you know why? Because daleks are **censor** fictional. They don't exist in reality, therefore you cannot actually be one. Nor can you be anything else besides a human, because that's what you are, you aren't a cactus, you aren't fridge-kin, you're a human.
Note: I have nothing against pretending to be these things, that's roleplaying, that's for fun. There's a difference between pretending to be a thing and believing you are that thing.
This was a good vent. I feel better.