Number one rule: People.
You are writing, first and foremost, people. Not plot, not romance, not anything else - people.
People who have different views, beliefs, and even differently wired brains.
Talk to people with these different views and beliefs and neurotypes. Keep an open mind. Learn from your mistakes.
I've recently made an autistic character, Analise. She's 18, lives with her mom, and fixates on glass and color. To her, rainbow arrangements of color - the way the light moves through colored glass - that is something so unbelievably amazing and fascinating. It'd be like me being able to see a glory of unicorns or group of dragons.
I've taken this from reading a crapton of autism blogs, regularly getting advice from a fellow roleplayer who also happens to be autistic, and from finding Analise. My goal may have been to create an autistic character, but I'd still have an "Analise" if I took away her autism. An Analise who might have less trouble getting words out and won't have her fascination with blown glass, but still an Analise interested in learning.
When you're writing a character with a different world view than you, you need to realize that. When you don't have depression, you need to make a person that can be separated from their depression: a different person, but still the same.
This might sound contradictory, but life's experiences are what makes us. If I wasn't anxious and depressed, I would be less empathetic and emotionally invested in others. I would be more outgoing. But I would still be me.
Or we can go deeper.
If Analise wasn't autistic/didn't have autism (whichever you prefer), she would have been bullied for different reasons (because, let's be real here, everyone gets bullied at least a little), she would never find her love and fascination she finds in glass decorations and with prisms and light, she would be a different Analise.
If I didn't have depression I would be a lot more outgoing. I wouldn't see or recognize people that didn't have as many friends. I would sit at busy tables and never have my times alone in quiet introspection. In short, I would be entirely different.
That is the precarious balance you must maintain: people. Make people. People who are the way they are because of their experiences in life. People who are people because of their hopes and dreams and experiences.
Don't make the disease or different neurotype or whatever. Make a person who has it. Who could have that particular thing taken away and be themselves, but would also have lost experiences that make them who they are.
So many people would be different.
You wanna know the most important thing? When you're trying to make someone different? Who maybe has stigma because of what they were born with, what life gave them?
DO NOT PERPETUATE STEREOTYPES.
Don't make a violent schizophrenic or a depressed person who is actively depressed in a social situation surrounded by veritable strangers or an autistic way into coding and programming or something off the wall.
You embarrass me.
Make a schizophrenic who forgets to turn off the oven because they think someone else has got it, or a depressed person who confides in their closest friend, or an autistic person infatuated with writing. Make someone who has one or more of these issues and is a famous doctor or a famous scientist or even a politician.
That's the thing.
You can overcome whatever life throws at you. Nobody is a stereotype.