Derangements can, and should be, tricky things. Sometimes I believe that people become focused on the specifics of their character and forget to take a step back and not only think about how their character does things or what the character thinks, but also how their characters thinks about thinking about things. It is my supposition that most derangements fall under one of two broad categories
Altered perceptions of the world
Thinking about which of these two categories a character’s derangement falls in may help to “get in their head.”
Let me give you some examples – paranoia, anorexia, gender-dismorphic disorder, some types of schizophrenia, habromania, necrophilia could all be described as viewing the world in a different way. These characters react to their world in a, relatively, logical fashion. If the world was truly conspiring to get you, then it would make sense to trust no one. If someone was fat it would make sense to diet and lose weight. It someone was born in the wrong body, was being attacked by ghouls, found things funny or if dead bodies were dead sexy, then the responses of these characters would be reasonable.
Alternatively OCD, manic-depression, depression, and crimson rage are generally uncontrollable thoughts / emotions. The need to count things, knock a number of times, check on the stove. The feelings of sadness, giddiness or rage are uncontrollable.
Obviously these are not hard and fast rules, perhaps a character doesn’t believe that the world is out to get them (paranoia) but is “merely” scared all of the time and cannot control that feeling. Alternatively she may not believe the world is out to get her, but that damn voice in her head (literal or figurative) asks her is her childe doesn’t really just want to diablerizie her when she’s weak. This voice may be a literal “external” voice or it may be that voice that people have when they talk to themselves or think about a topic.
I’ve already gone over my word limit, but let me offer one last thought on this topic. Once you decide which of (or both) these broad categories your character falls in then the question is how does she feel about it.
If she lives in a world where dead bodies are gorgeous, why don’t other people feel that way? Is she so confident of her point of view that she scoffs at others who disagree? Does she wonder if there is something wrong with her or perhaps that it’s just something no one else talks about in public but everyone does it?
Are the uncontrolled thoughts something of a frustration, one of the definitions of insanity is partially defined by its affecting the ability to have a normal life, or is it a part of the character’s life that they’ve accepted. Getting to an appointment at midnight just means that you have to get ready at 8:00pm to make sure that all the doors are locked (three times), the stove is off (even though you’ve never used it), you only take right turns.
Think about thinking about how they think about thinking about it.