So for what I'm writing I have a Character who slowly descends into madness. However, I want it to be subtle. So she starts off completely sane and as time goes on, subtle hints at how she is slowly become more and more unhinged. She isn't a big character but she's important to the story. How would I do that? I think I have a rough Idea. I'm just looking for some inspiration on how it can be done?
'Sanity' and 'madness' are pretty vague terms, frankly. Does she have a psychotic break? Is she falling into depression or mania, is she suffering from schizophrenia or OCD or PTSD? There are a lot of thing to consider here. I'd think about what she's specifically going through and do some psychological research, find out how these diseases most commonly exhibit themselves, and take inspiration from that. If you're not willing to do that, my advice is don't write this character. We don't need any more vaguely-described 'mad' or 'insane' characters who exist just for their ~madness~ to further the plot.
The above person is totally right that it's important to be accurate. There's a lot of different types of mental illness -- bipolar, schizophrenia, and plenty of other ones that might not seem as clearly "crazy" to outsiders on the surface. You've got to do your research on different types of disorders to pick the one you'd need, and do it correctly. Study the up-to-date DSM guide - that has the criteria that licensed psychologists use to diagnose. Once you've got that part nailed down, I'd also make sure to characterize the person's "oddness" by their body language, mannerisms, and other subtler clues as opposed to just their words and big-picture actions. Also, if the character has an issue that involves instability or manic episodes, you could have them be drastically different to their "typical" self in order to be clearer that something's off with them. If she's becoming unhinged gradually due to an issue like grief or delusion, you could also consider having her become increasingly fanatic over an absurd religious or conspiracy belief.
I personally don't think a look into their psyche is needed to tell the story, if for one the character doesn't know she has anything wrong with her, in my experience most people with the above mental health problems don't even know or don't think they have a mental health problem and could have several traits of different mental health that can't be defined. In that sense it might be a good idea to look up some info on mental health problems but you don't have to be specific to one mental health problem. The first signs could be anything but if you want them to gradually get worse then bad health would be the first, they won't be taking care of them selfs as much, they get agitated because of possible lack of sleep, they look tired and unwell, pale and then gradually get worse, they hide away from the world and could get upset over the littlest thing. This is just things I have noticed, i would still recommend to read up about it.
I very much disagree. There needs to be internal consistency. There needs to be an understanding of what illness the person has, so that their characterization isn't just a slew of plot-convenient symptoms, which is what you're describing. It's a complicated issue: chemical imbalances that produce generalized symptoms like you'll find listed in the DSM filtered through an individual's experiences and personality. It deserves to be given a thoughtful and respectful look, not simplified to "they stopped taking care of themself and just got upset over the littlest things" - even if that's how other characters perceive it (which would be unfortunately realistic), the writer needs to know better. You should understand your characters' motives and thoughts, and there's no pass on that just because the character's mentally ill.
My book is a tragedy 'cause I love them! So to make it tragic I also have to send my person into 'madness' or just make the ending very sad. My character, Lu, is very conscious of how people perceive her and she wants to do no wrong yet seem heroic and she is only really good at healing, so to send her into a downward spiral I take these things away from her. People start picking her apart and seeing her inner qualities and the more she tries to hide it the more she reveals. People also start falling ill and dying which pretty much makes her feel worthless. Izzybot though has got it right, you can't just say someone is mad as that's not really a thing, though it is quite hard trying to diagnose a character.
That wasn't what I was saying, of course the initial way of writing the progress needs to be described properly not just a simplification that they "stopped taking care of themself and just got upset over the littlest things!" Why would you even say it like that? I was under the impression the writer wanted advice on someone gradually getting a mental health problem and as I have worked for a long time in the Mental Health profession I was just explaining the first stages that I have seen first hand. I guess we have different opinions! I would still recommend to read up about it as stated in my first post!
That was literally paraphrased from your original post. Maybe I misunderstood you. My disagreement was with your comment that it doesn't need to be "specific to one mental health problem" hence my comments about internal consistency. IMO a writer needs to be driven not by what other characters and the reader can see (symptoms like depression and lashing out), but by what's actually going on inside the character, so they need to actually understand the illness - not just throw in a grab-bag of symptoms. Like @I.A. By the Barn said, diagnosing characters can be difficult, but that doesn't mean you shouldn't do it.
