I don’t really get the appeal of the whole “autistic headcanon” thing. (Meaning, discussing how a fictional character is or could be autistic when it’s not explicitly stated in the work.)
I think there are two reasons why.
1. I’m not all that aware of how I act and how I seem to other people, so I’m not likely to notice if a character outwardly acts or seems like me. I’m particularly lacking any sense of what my body looks like. I’m intellectually aware of some specific weird things I do, and I assume my body language is weird in general, but I don’t actually know what my facial expressions etc. look like. So I don’t really notice autistic-ish body language in others either.
2. Comparing myself to a character that way is uncomfortable for me on some level. Like seeing a picture of myself that’s taken from an angle I can’t normally see. Even if it isn’t unflattering, it’s unexpected, and it takes an uncomfortable mental adjustment to fit it into my self-image. When I do happen to notice that a character acts like I do, a lot of the time it feels like something private about me is being revealed in public. On the whole it’s probably good that I know what I look like, but it’s difficult to swallow at first.
I’m a lot more likely to relate to a character based on how they think, if the story is told from their point of view. But it seems like stories like that– that portray autism or literally any brainweirdness in the main character from their point of view– are a lot less common than non-POV characters that could be interpreted as autistic. Fanfic is pretty much the only place where I have characters I relate to because of brainweirdness.
And the truth is I don’t tend to compare myself to characters much at all. I guess I just tend to be too outside myself while reading, too focused on the characters’ thoughts and feelings rather than my own. The topic/trait/whatever has to be very personal and very emotional for me before the comparison has more importance to me than the character’s perspective in the story.