[quote=Kestrel]
To realistically write a character with mental illness, you must understand that said condition is a core element of who they are and not just an add-on. It affects how you think, perceive, feel and decide every day, every waking minute. To this end it's advisable to do a lot of research to make a believable character, as well as try to keep the number of conditions low as one by itself is often already crippling. For example, a person with ADHD lacks the focus to write a long, coherent response. A character with PTSD is going to be banned from any kind of combat scenario, etc. That's stuff you have to think about when you're writing your character. The more you pile up, the less likely a functional, or even breathing individual comes out of it for you to RP with.
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Coming from my own experiences, definitely. Mental illness isn't something that's all "oh, he just happens to have it," and it's something that really affects the quality of life of a person. Unless I decided to try something out that I don't have personal experience with, in which case I would do my research, pretty much all the mental illnesses I play out with my characters come from my own experiences with what I have.

Though also from my own experiences, there's also going to be characters that have varying degrees of the severity of their afflictions, or have otherwise learned to cope with their afflictions. If I didn't learn how to cope with all the shit that I happen to have, this thread would have been about how the Mossad are using the RP Guild to brainwash me into communism to make way for humanity's enslavement by the Intergalactic Reptilian Empire at best. At worst, I wouldn't have even been able to join the RP Guild in the first place. Though in which case, the mental health issues would be hidden underneath the outer layers of the character, and not really worth bringing up in a character sheet or other form of introduction. Rather, they would be revealed as the story progresses and the character has the outer layers of their personality peeled off by whatever experiences they go through in the RP that would cause their inner core to be exposed.

You've met one person with autism, you've met one person with autism. You've met one person with PTSD, you've met one person with PTSD. Likewise for everyone of not just every mental affliction, but also race, gender, sexual orientation, religion, non-religion, and the list goes on. We're all individuals, simple as that.