Psychological disorders are often poorly handled in fiction and film. When I was in graduate school and would go to see horror movies with non-psychology friends I would often be questioned as to: "What's wrong with that dude, man?" I appreciated their need to know, but the question was often impossible to answer because either 1) the "dude" appeared to be suffering from at least half a dozen disorders simultaneously, or 2) the "dude" was possessed by some evil entity as yet unknown to science. At the same time, I generally found that fiction writers did it much better than screenwriters, but still would make routine and disconcerting errors. It finally got so bad that I simply stopped watching movies of "psychological" interest. To this day, I seldom watch a movie that turns on some psychological principle, such as schizophrenia or multiple personalities. I do still read "psychological" books, but they are not my preferred reading.
So what should a writer do who wants to use an element of psychological deviance in their fiction? Well, one thing you could do is read an article by Tracy Knight in On Writing Horror called "More Simply Human." This is one of the best short pieces I've ever read on the subject, and clearly Ms. Knight knows her subject. She certainly points out one of my particular peeves, how many times writers confuse Schizophrenia and Multiple Personality disorder. They are not the same thing and have far different causes and courses.
Here are some other things that Ms. Knight either did not cover or only talked about briefly.
1. "Crazy" is a vague term that is almost never used by psychologists.
2. The "Insanity" defense is a legal concept and does not correspond exactly to psychological descriptions of insanity.
3. Having a diagnosed psychological disorder does not mean that you will always have it. Many disorders can be cured with little chance of relapse.
4. Psychological disorders come in all levels of severity. Some are quite mild while others completely disrupt the person's life.
5. A person with a psychological disorder, even a severe one, is not necessarily going to exhibit the symptoms of that disorder 24 hours a day. They will usually have periods where the disorder appears to be gone or in remission.
Finally, it is my belief that if someone filmed any of us 24 hours a day for a year or so, and then picked just the right fifteen minutes from that film, they could make any of us appear to be suffering from almost any known psychological disorder. In other words, you ain't so far from crazy yourself.