Let me begin by saying that even having a degree in psychology, writing about a character's mental disorder convincingly is extremely difficult. With that being said let us consider the idea of malingering, which is essetially what we will all be doing as we compose these expositions. Some of the most restraint I have noticed with younger or more novice writers is the lack of research. Research in this case is paramount. If you want your reader to believe that your character truly is afflicted by some level of mental disorder you need to deeply understand what that means and attempt to harness - if only for a short time - how it might actually influence your day to day life having Disassociative Identity Disorder, being Schizophrenic or even having something as common as ADHD. It might be more important to ensure that you get the information correct for the current time frame that you are writing for. Something like schizophrenia would not be responded to in the same manner during the 15th century as it would be in the current century. On that same note, it would be a wise idea to do your research into treatment or medication if that is necessary for your story. More importantly make sure you grasp the side effects of those treatments. The truth is in the details. If you want your character's affliction to be believeable you need to make sure you understand what is going on with your character's mind and be specific.