This post is going to be a little more anecdotal than usual, and also ranty, but bear with me.
A little while ago I read a book and found one of the non-protagonist main characters was said to have bipolar disorder. I started cringing but tried to keep an open mind.
I basically didn't stop cringing.
It was one of the more offensive portrayals of mental illness I've read in recent memory, perhaps because it seemed the author tried to humanize this character. The trouble was that she was clingy, volatile, irrational, conniving, paranoid, and downright abusive to the one person who was supposed to be her friend (which other characters pointed out). That would be fine--lots of people are like that, including bipolar people--if the explanation given for her behavior didn't boil down to, "she's off her meds."
Hey, sure, when I'm having an episode (which, contrary to fictional depictions of the disorder, are just that: EPISODES--they occur between periods of fairly stable moods), I can be irrational and volatile and paranoid. But the core of who I am--loyal, compassionate, smart, creative--is still there as well. I don't suddenly start drowning kittens when I'm manic or depressed.
ALL of the horrible abusive traits this character had--the reader was told repeatedly in the text--were symptoms of her bipolar disorder when she was off her magical meds. Bipolar disorder figuratively turned her into a monster who was possibly capable of murder.
I am willing to bet money that the author looked up bipolar disorder on Wikipedia, read a few possible characteristics, and left it at that. And if I can get pedantic for a moment, one of the reasons I believe this is because the character was supposedly on unspecified "antidepressants", which five minutes of reading or any doctor will tell you is a risky proposition (had the author said "mood stabilizers", I would've given her the benefit of the doubt). Further, nothing about the character suggested she was depressed or manic; while she was "off her meds", if anything she came across as being in a constant mixed episode. Probably because the plot required it, not because the writer knew what a mixed episode even is. The writer could have, in fact, simply passed off *all* this character's personality flaws as "because she's on her period" and it would've been just as believable.
Julie, in her last post, absolutely has an excellent point about not getting caught up in the details; my caveat would be that you absolutely have to know in detail what you're talking about when you are portraying people who could be damaged by your depictions of them.
Don't take on mental illness if your only exposure to it is the bad guy of the week on Criminal Minds or Wikipedia. Spend some time reading from the perspective of those you're writing about, even if they're secondary characters. Diversity is important, but so is NOT contributing to stigma.
As writers, we often say the story is most important and as long as that gets across, it's okay to fudge certain things. But "it's just a story" is not a defense when you're contributing to the overall narrative that makes life more difficult for marginalized groups. Daily, people like me have to push back against the idea that our mood disorders makes us dangerous, and that the only way to function and not be violent is to medicate (and that pills are magical). Mine is an illness with an 85% survival rate, in part because it's so isolating and stigmatized, which makes it extremely difficult to reach out for help. Fiction that contributes to stigma and inaccuracies stacks more weight on everyone's shoulders, and it makes for lazy fucking storytelling.
The "story" does not exist in a vacuum; it exists in a world where all things are not equal.
It's your responsibility to educate yourself about people in the world around you and to question your own depictions of them. It's on you, the writer, to get shit as accurate as possible and make an ATTEMPT to know what the fuck you're talking about.
And editors? Beta readers? Same thing to you. Because I've no doubt half a dozen people saw this book (and books like it) before it was published and not a single person called out an offensive depiction of mental illness. Whether it's a plot hole or inaccurate medical information, you should be asking the writer to know what the hell he or she is talking about as well.