The internet and sites like Webmd have allowed ordinary, non-professional people to become informed about medicine in an unprecedented way. Patients can now bring printouts to their doctors, confident that they know what they've got and how to resolve it. This has become even more true lately with the rise of television shows and books that de-stigmatize and show the details of various mental disorders. Thanks to shows like "Obsessed", "Addicted", or "Hoarders", we know the symptoms of obsessive compulsive disorder, for example, and what methods are used to treat it. And one odd effect of this newfound knowledge is that medicine and psychology has been creeping into our fiction.
Many people write about characters with some sort of mental problem. In the past we would just wing it, and let the symptoms shape themselves according to the character we were creating. Now, though, there's a temptation to check behaviors off an acceptable medical list. Washing hands excessively? Check. Inability to make decisions? Check. Lying to loved ones to hide an addiction? Check. All the guesswork of unstable behavior has been taken out of the equation - and sometimes I worry that it will make our characters seem more like case studies than with people.
Don't diagnose your character
The problem with this approach toward character psychology is that you will end up diagnosing your character rather than exploring him or her as an individual. It's important for doctors to systematize symptoms and see people as a little less than individuals. After all, the only way we know how to treat people is by seeing how their symptoms are similar to others' symptoms. But that's not the job of the writer! You've got to look beyond the clinical assessment of your character and give him his own life.
Imbue your characters with human problems.
If you are writing about a character with mental problems, consider the example of a very humane doctor, Oliver Sacks. In his books, Sacks considers his patients' stories not just as the story of the disorder, but also the story of the person's life before and after the disorder. He discovers what they love and value in life and tries to help the person return to those things and people. Often forces like music or art can restore a person's sense of self. It's this very humane treatment of patients that you can emulate in your writing. Your character deserves to be an individual with his own loves, desires, and disappointments that aren't entirely dictated by the confines of the disorder.