A person who is not sane cannot pretend to be sane. It is equally difficult to pretend to be insane if you are sane. There is a scenario, known to psychiatrists, in which a person believing himself to be sane, presents himself as insane to the doctor, but in fact the person really is insane! He thinks he is faking, but he is not! Thing is, in order to know how to act insane you almost have to be insane. There is no way to understand serious mental illness unless you have it yourself.

the doctors have a special test -- they take you to the bathroom with a bathtub full of water, give you a spoon and a ladle and ask you to make the bathtub empty