The first autistic character I ever read about was a girl named Susan in a children's book. She was a piano savant who could play anything she had heard, even if only once, though it was plain she did not understand the music; when she heard a record, she copied the skips of the record player too. She didn't communicate, didn't have friends. At the end of the story, Susan was sent to an institution, and everyone agreed that it was in her best interest. (Anne M. Martin, The Secret of Susan.)
It was published writing. It was also very bad writing--and not just because autism wasn't depicted in a realistic fashion. The biggest problem was that Susan herself was not a well-written character. She was not, in fact, a character at all; she was a plot device. She did not change or learn; she reacted to events as though pre-programmed with her responses. She was not human.
So let's talk about this. Let's talk about good writing, and about autism, and about why characters like Susan need thorough re-working.
1. Your character is not a walking blob of autism.
When you write a neurotypical character, you have to write them as a complete person. That doesn't mean giving every single minor character a complete backstory, but it does mean that what your readers see of your characters should be parts that fit into the larger picture of a whole person. Autism is a part of that picture for your autistic characters, but it does not make up the entirety of that character. If you take a list of diagnostic criteria and use that list as the description of your character, you will have a computer program, not a person. In the real world, autistic people don't act like a list of diagnostic criteria; they act like people. Most of their traits are traits not on the diagnostic description, and most autistics will be exceptions to the rule on one or more traits. For example, there are autistic people who have wonderful imaginations, who speak fluently, who are extroverts, who are popular, or who really, really suck at math.
2. Your character makes their own decisions.
When a neurotypical character takes an action or makes a decision, there are multiple causal factors. What they do is up to them, and it's determined by their personality, by their mood, by their relationships, by the things they know. That should also be true for autistic characters. If an autistic character does something, it should never be explained that they did this thing "because they have autism". That's about as worthless a description as saying I did something because I'm an American. Unless it's something explicitly based on the diagnosis, such as, "She goes to a special school because she has autism", you'd better have a reason for your character to do things.
If your five-year-old autistic character has a meltdown, it's not "because he has autism"; it's "because his mother made him wear that itchy sweater on a day when he was cranky and hadn't gotten enough sleep". If your autistic scientist snaps at a co-worker, it's not "because she's autistic", it's "because she was preoccupied with her work and irritated to be jolted out of her concentration". If your autistic character wanders, it's because they've got somewhere to escape from or somewhere to go. If your autistic character rocks, flaps their hands, taps their feet, chews on pencils, or fidgets with a Rubik's cube, they do those things for a reason. And if you don't know why, then you need to go back to the drawing board, because you don't know your character well enough.
3. Autism affects, but does not determine, everything in your character's life.
Autism affects everything about a person because it is part of what makes them who they are. At the same time, autism does not create some kind of pre-written script which your character will follow. Think of autism as a trait along the lines of culture, gender, or nationality--it touches every life experience, but it does not prescribe life experiences. Your story about an autistic person should be about an autistic person who is also many other things--a part of their family, their community, their nation, their race, or even their species and home planet. Think about this: How does autism affect the way they see their role in their families? Their careers? Their country? And how do those things affect their perspective on their autism? And, most importantly, how does their personality mesh with all of that? Which brings me to:
4. They have a unique personality.
If you can write down "Autistic" as your character's main personality trait, you're doing it wrong. Autistic people are too diverse for "autistic" to be a valid personality descriptor to begin with; and besides, it's a cognitive trait, not a personality trait. Autistic people are more likely to be introverted than average, it's true, but there are extroverts, too. Some are angry and brash, others shy, others confident. Some are know-it-alls, others unsure of themselves. Some would happily deliver a beating if they thought the other person had it coming; others can't conceive of the thought of violence. They have diverse interests, goals, preferences, and desires. You need to do some serious thinking if you cannot write about who your character is without using the world "autistic" or describing autism.
5. Savant skills and splinter skills: Use with caution.
So you want to give your character a savant skill. Why? Because they're autistic and you associate that with autism? Because you think it's cool? Because it turns them into a budget superhero?
Before you give your autistic character savant skills or splinter skills, or even unusually strong talents, think about why you're doing it. What purpose do these talents serve? Will they serve to further the story? Or are you just randomly tacking them on without any real reason to do so? Even worse, are you tacking them on so that you can talk about the savant skill rather than the person? If your autistic character has devolved into a display case for their savant syndrome, scrap it and start over.
About ten percent of autistics have some kind of splinter skill, and they're usually not all that flashy. Not too many of us are prodigious savants; for most of us, a splinter skill is more like a knack or a very specific skill that we learned with unusual ease. If your autistic character has a savant skill, think about what that means to them: What does your character think of their talent? Do they enjoy it? Do they show off? Or do they hide it for fear of seeming odd? How aware are they that others can't do what they do without a lot of practice? Does it help them in their daily life, at work, at school, or dealing with other people? Might they use it as an escape from the hassles of life, or a springboard for a career? Does it give them a perspective that other people don't have?
6. Autistic characters are not to be used as a plot device unaffected by the plot.
All too often, I have seen autistic characters used as a source of conflict. An autistic child creates problems for their family. An autistic witness creates problems for crime-scene investigators. An autistic person sits at the center of the conflict, unmoving and uninvolved, while their presence diverts the story; but the story does not affect them in return.
Writing a love story between a single mom with an autistic child and her potential dream guy, and the autistic child is the problem they have to overcome to get together? That's a bad idea. That autistic child has their own perspective on things. Suddenly Mom is dating; there's a new man around. What now? The child will have their own thoughts on the matter. Or that autistic witness who can't communicate very well--they just witnessed a crime, probably a murder because that's what writers like to write about; a person doesn't walk away from that unchanged. Your autistic character is not a black box from which the investigator must extract information. They have their own story.
7. You can't group real autistic people into "Smart" and "Stupid" types.
Fictional autistics come in roughly two categories: The "smart" autistics, who are stiff and formal and genius at something, usually something scientific, but who take things literally to the point of obtuseness, and the "stupid" autistics, who need lots of care and have a splinter skill they don't quite understand themselves and occasionally show that they're more perceptive than you think. But autistics are more than Spock and Rain Man, and you can't stuff them into those boxes. Each person has their own particular set of skills and talents.
8. Their morality is human morality.
An autistic person is no more likely than the average human to be either a saint or a sociopath. Autistic people have trouble understanding and communicating emotion, but they have as much compassion as the general population, and their ethical decision-making is very much like that of any other human being. If there is an exception, it is that autistic people find external, specific rules easier to follow and take longer to figure out when rules can be broken; for example, your autistic character might drive at the speed limit even when most cars are going ten miles an hour above it. But even then, some autistic people are rule-breakers, and many, especially adults, have their own systems of values that they follow regardless of the law or social convention. Your autistic character will care about the same things most people care about--the well-being of their family, their loved ones, their friends, their country, about fellow human beings, animals, life in general, the world in general; about knowledge, cultural traditions, religious beliefs. Their atypical cognitive style may lead them to act on those ideas in their own unique ways--but their beliefs run the same human gamut as non-autistic people's do.
9. Your autistic character has a future.
Many of the fictional autistic characters I've encountered end up in one of three ways: Cured, institutionalized, or dead. Something about the writer's need to tie together loose plot threads seems to demand that autistic characters' autism be resolved somehow, that an autistic character cannot simply go on living their life, being autistic. Ditch that. Your autistic character does not have to be cured, nor do they have to die, nor do you have to "send them away" somewhere out of sight and out of mind.
10. All other rules that apply to good writing, still apply.
Why is it that people think that once you're writing about a disabled character, you can ignore things like avoiding melodrama, providing complex motivations for your characters, or remembering that everyone is the main character of their own story? If you find yourself doing this, back away from the novelty of writing about an autistic character and think about your character's preferences, dreams, goals, pastimes; think about the people they love, the places they call home, and their image of who they are. If you are writing about an autistic character, you are by definition writing about a person. "Autistic" implies "person", just like "woman" or "American" does. All of those human universals still apply.