I think it's important to read personal accounts *by* people with that particular mental illness -- not just from their buddies/families/etc.
What was said should be taken as good advice for accuracy.
I'm a paranoid-schizophrenic. I was diagnosed when I was 17 years old. I've been through the hallucinations, the voices, the beliefs in the craziest things (CIA communicating through dental fillings wouldn't be too far fetched during an episode).
I think that if you were going to portray someone with a mental illness, that you should not have to use those symptoms as parlor tricks or as special fx for your plot. I don't think it would be easy, and it would be more debilitating for the character suffering from it.
Another thing to keep in mind is that people who suffer from these type of illnesses might not be like your average citizen of America. I learned from my case manager (a very good one by the way) that a large majority of schizophrenics are not very successful in life. I'll take a guess at what I recall and it was over 50% of schizophrenics who are unsuccessful. They continue to abuse drugs and are poor. Suicide rates are very high among people with schizophrenia. The other smaller percentage (maybe as low as 5% but as high as 10%) are your regular working citizens who earn money and can support themselves. One of my early psychiatrists is a schizophrenic. Magazines for schizophrenics highlight other successful people suffering from this disease. But the truth is very disconcerting. I didn't want to believe it, but I accepted it. I may be one of those unsuccessful schizophrenics. At one point during my worst episode, my voices suggested that I kill myself. At the time, I was suffering through many months of panic attacks every day and night. I lived in fear during those days, worse than what any horror movie could put an actor or actress through. I couldn't sleep. It was hell.
Years ago, I was hospitalized often due to the schizophrenia, which was why I quit my job as a sound engineer. I literally lost my mind. I had many unpaid hospital bills. I couldn't remember even my name. I could hardly speak words and understand what people were saying amidst the chaos in my mind. That's how bad it was. To portray someone with symptoms like these would be very difficult. I can say with confidence that living with a mental illness is like living in a poor world. Literally. I'm on SSI and food stamps. I have a criminal record due to my untreated schizophrenia as a teenager. I saved up money to pay those hospital bills. If it weren't for my father, I would have to go into public housing, which is very difficult to get into in the state of Oregon. I'm what you consider a very fortunate case when it comes to getting the help I need. Others aren't so lucky. I've met other schizophrenics in my group meetings with my case manager. Seeing is truly believing.
My Dad has OCD and I have met another person with OCD in my group meetings at my case managers office. To me it doesn't seem like a very "awesome" mental illness to have.
I spoke about myself to give a better idea of what a real person with mental illness might seem like. I'm a border line between successful and non-successful. I have an opportunity to go back to school soon and hopefully get a certificate or degree in something so I can get a job. Life isn't easy. I might not pass the classes. I'm fortunate in that I can write as well as I can. I'm currently working with the Offices of Vocational Rehabilitation -- a state run place that helps people with disabilities get a job.
Portraying someone with a mental illness should be researched as much as you would a court room drama if you don't have any knowledge of it. Reading the paper work is one thing, but actually seeing these types of people can really give you a different perspective on the character you want to write.
If you have the time, I advise you to go onto Amazon.com and search for the novel
Halfway House by Katherine Noel. An excerpt is available on there and I suggest reading it. I read it and I was shocked by the gritty reality of one of the characters. I haven't read the entire book though. In my opinion the writing is excellent.