In order to learn something you are dying to know about, you can't just make reasons based on pure scientific evidence, you have to also ask them and get to know the person, even if the person is hiding something. Just by speaking to them, you can get an idea of how they are hiding themselves from you or others. Even if the person is just quiet and having a poker face, that pretty much means the person has that type of personality.
With you asking all those funny questions or offensive remarks just to see how the poker face person acts, gets pretty interesting. If he or she gets all crazy on you, then might as well call the cops for emergency backup.