This post is a bit longer than my usual 3 minute read, but it was important to go into detail in places.
Whether you suffer with mental illness or not, the mentally ill character is often a trope that, if done badly, can come across as offensive and cartoonish, and give actual real people with that illness a bad reputation.
As writers, it's important that we represent people correctly (for good or ill) and make the character a real person, and not just a twisted parody of one.
In this post I'm going to talk about how to avoid that. Please note that of course this is a sensitive topic at all times, and is just as subjective as any other writing tip.
It wouldn't be possible for me to cover the entire spectrum of mental health problems, but I'm going to cover some of the main or perhaps more obvious ones here.
Symptoms matter - do your research
The DSM-5 (Diagnostic and Statistical Manual of Mental Disorders, Ed. 5) is the BEST source of information on all mental health problems, as they are - at the present time - more or less irrefutable. PLEASE do not rely on YouTubers, blogs, tweets or similar opinion-based sources when it comes to research. People may not explain their own symptoms in a way that does them justice, and you run the risk of misinterpreting them. Also, people do lie and make things up.
If someone went to the doctor suspecting Type 2 Diabetes, they might have tingling feet, increased thirst, increased urination and heavy fatigue. Maybe they have dark patches on their skin, cuts that won't heal, and so forth.
These are what we call textbook symptoms, and all illnesses - including mental - have them. Even the more unusual symptoms are 'textbook'. If they're not literally in the textbooks, that person is either an outlier, or has some other condition they've not been diagnosed with. Also check the typical age-range of an illness. If it's not heard of to suddenly develop it at 25, don't do it.
The rule of thumb is if your character is experiencing a symptom - MAKE IT TEXTBOOK or leave it out. Don't EVER borrow symptoms from illnesses they do not have, just because it helps move your plot along.
As I said above I couldn't possibly cover ever mental illness in detail here, but whatever illness you choose to give them, remember that most illnesses have different sub-types, which in turn have different symptoms.
Understanding the huge spectrum of mental illness
NOTE: Again I cannot possibly go into every mental illness, but what I hope to cover here is the most common ones that you tend to see in popular media and fiction.
BIPOLAR DISORDER
I'm covering this one in more detail as it seems to be the "go to" for characters who are violent, unstable and generally not-very-nice. Whilst it's silly and irresponsible to deny the 'bad' sides of this illness, they are not always the most displayed traits, and sometimes don't happen at all.
Did you know there are FOUR types of bipolar and they're all completely different? If your character is bipolar, research the four different types and decide which one makes sense for your character. Even if they are undiagnosed - the symptoms are there regardless of the label slapped on them, so look it up.
BIPOLAR I: This type has "manic" episodes that last over a week at a time. Mania is not a good mood, but the person can SEEM to be in a good mood. It can result in being chatty, not sitting still, being a bit 'much' to deal with, etc. It goes unnoticed in younger men, as it's seen as normal behaviour for them quite often.
Sometimes the sufferer will experience hallucinations. This is the type of Bipolar that often gets turned into the violent stereotype, as during a bad manic episode, people are often a danger to themselves and others, due to how aggressively excitable they can become. Immediate hospitalization is sometimes required, and the Police can be called during a bad manic episode. However, the 'danger to others' element often comes from a place of fear and being unable to control their emotions, not because they have an inherently violent nature. They're not psychopaths, they're not going to become a serial killer because of it, for example.
BIPOLAR II: This type does not have the mania spoke of above (the mania is much milder and short-lived), and is a much more depressive disorder that results in a lot of fatigue and 'down' time. There can be hallucinations with this type, as well. This is, essentially, the "opposite" of Bipolar I.
CYCLOTHYMIC DISORDER: This is a very mild version of Bipolar where the person has regular cyclical mood swings, but not quite so 'dramatic' enough to qualify for Type I or Type II. This can develop into Type I or II later if untreated, however and is sometimes considered to be an 'early' form of Bipolar.
SUBSTANCE ABUSE INDUCED BIPOLAR: Frequent serious drug abuse can cause permanent mental illness due to the damage done to the brain (for example, it can cause schizophrenia as well). A person with this type of bipolar may in turn have any one of the above types, the difference being it is purely due to their addiction that the illness formed.
SCHIZOID/SCHIZOPHRENIA
This is one that is often the most used and abused in fiction when someone needs a "bonkers" character. Schizoid and schizophrenic disorders are an absolutely HUGE spectrum involving various diagnoses, symptoms and behavioural traits. These are often under the umbrella of "schizoid disorders"
I'll list a few here that you can Google if you're thinking about making your character on the schizoid spectrum, with the different sub-types in brackets. You'll see how how deep these diagnoses go.
Schizophrenia is NOT having multiple personalities, and big Beautiful-Mind-like hallucinations don't happen. If one suffers the appropriate type of disorder, one might have big, grandiose delusions that they are - for example - a Secret Agent, and have shelves full of nonsensical notebooks, but they won't see entire cinematic scenes before them like in that movie.
Schizoid Personality Disorder
Schizoaffective Disorder (Bipolar Type/Depressive Type)
Schizophrenia (Paranoid/Catatonic/Hephrenic/Disorganized/Residual/Undifferentiated)
Much like bipolar, schizophrenia can be induced by long-term severe drug addiction such as cocaine. It can also be present in childhood and disappear later in life, or come on as 'late onset' illness in adulthood.
Now the important question: can a schizophrenic get violent? Absolutely. I had a landlady whose drug-induced-schizophrenic son attacked her because "she was coming after him with the man in the garden" (a hallucination). But much like above with the bipolar disorder, it's often out of fear; a schizophrenic's delusions can make them believe terrible things about their loved ones and it may cause them to lash out. They are not, however, inherently violent.
DEPRESSION
This is one that you may not realise you're misrepresenting. Depression is often spoken of as if it is one thing with one cause that never goes away, but it's not. There are many types of depression. Again, as above, check your character's textbook symptoms to make sure you're representing them correctly, and not borrowing symptoms from other types or disorders.
Major depression
Persistent depressive disorder
Psychotic depression
Peripartum/postpartum depression
Situational depression
Premenstrual dysphoric disorder
ANXIETY
Again, there are multiple known types of anxiety with different symptoms, treatment plans and reasons for existing. Don't just make your character "anxious", figure out why and how it typically manifests.
Generalised anxiety disorder
Social anxiety disorder
Panic disorder
Mild phobias (spiders)
Severe life-changing phobias (agoraphobia)
Separation anxiety disorder
THE NEGATIVES OF MENTAL HEALTH: WHEN TO ACKNOWLEDGE THE HARSH TRUTHS
The unfortunate thing about mental illness is that it's not very nice. As well as all the harm it does to the sufferer, sometimes, it makes the person not very nice to be around, either.
It absolutely can make them abusive (physically, sexually or mentally), cruel, vindictive, a liar, manipulative, and even violent towards other people. It's ignorant and wrong to pretend that these unfortunate traits don't exist.
However, it's absolutely vital to make sure that these traits are an actual symptom (remember: textbook!) of the illness you're trying to portray.
Having Mike murder his wife because of his OCD or Anxiety is just stupid.
But if Mike murdered his wife because he had a psychopathic disorder, then this wouldn't be completely unusual behaviour, since violent behaviour and a disregard for right and wrong are textbook symptoms of psychopathy.
Just remember: if it's not "textbook", they likely wouldn't have the symptom. Happy writing! <3