When I roleplay people with mental illnesses... I do something which is probably unforgivable. But, um, well, I do that.

When I play someone who is mentally sick, I do not go and research the mental illness. In fact, I never look up anything at all, not even the name of the illness. I don't even know what I am giving my character. When I create the character, I give them the illness that I want them to have. I give it the properties which I want to roleplay in my character, and twist it the way I want it to function. For instance, at some time I wanted to play a character which was an adult but with the mind of child. I did so, never learned if that was any specific name for that or anything. At another time I wanted to play a character completely unable to feel sympathy or sorrow, or anything else like that, and so I did. At another time, I played someone who got terribly scared and shy without wanting to the moment she even entered vision range of another person. Some of these I don't even know if they are character traits or mental sicknesses, I give my character what I want them to have.

More than once I've had my character whose mind I've twisted a bit be asked about "he has -name of mental illness-, right?"... And my answer is "Could be!". At times, I go check what it was that was referred to, and lo and behold, it was what I had given my character without even knowing the name of it or doing any particular research. I have yet to encounter a time I've been absurdly wrong in what I gave the character, since I never really gave the character that specific mental condition to begin with. I just gave them a mental condition of my customization, after all. Up to the rest to identify which name fits it the closest. Heh-heh.

That's what I do, anyway.

Edit: Of course, I don't mind people who go into detail researching exactly the illness they want for their character. In fact, I respect them, because I could never do the same. This is because, I would get too scared of insulting or misrepresenting actual people with said condition, and as so I shy away from actually calling any such condition by name, even if there is a perfect one for my character. That's just how I work.