Post History
Start by defining the insanity. Mental illness is a very very broad field: you need to narrow down what you mean by "insanity/psychosis"; give your character a defined condition, and write him with...
Answer
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/31584 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Start by defining the insanity. Mental illness is a very _very_ broad field: you need to narrow down what you mean by "insanity/psychosis"; give your character a defined condition, and write him with the correct characteristics and actions for that condition. You don't have to name the condition, or refer to it at all outside of your personal notes; just understand that insanity is not a case of just behaving randomly; there is always an underlying reason for any symptoms. Pick any insane or troubled character you can think of from literature -- Don Quixote, Hamlet, Moriarty, Scrooge, Eeyore, or many many others -- for all of the good ones, it is possible to read the stories and come to a definitive diagnosis of the character's mental illness. You need to write your character like that. If you don't, people simply won't connect with the character and will find them unbelievable.