I just don't know how to respond when I notice that a person is noticing that my mannerisms, facial expressions ect. are "off."
That's when it pays to rehearse what you want to say to an interviewer. And above all to consider it a "script" of sorts,
and to stick to that script like glue. No matter how the interviewer may look or make you feel self-conscious. You cannot necessarily hide how you look, but you certainly have the ability to control what you say. To limit it solely to the issue at hand- a job, and the skills you possess to get it done on a regular basis.
Most of us have all been there before. And no, it's never easy. Though the more interviews you get through, the more experience you gain in understanding what to do versus what not to do.
It's a process that you cannot go into with the notion of getting the first job you interview for. To be prepared to repeat the process as many times as it takes before finally landing that job.
One other thing to ponder. The legal nature of any job interview. That whatever you claim about yourself if likely to be followed up. And if not substantiated, can potentially result in termination. So before you ever confide to anyone formally interviewing you, know that you'd better have formal medical records in your immediate possession to back it all up. Otherwise it might amount to fraud in the eyes of a potential employer when it comes to formal accommodations.
As for putting your autism on any resume, I'd see that as a form of "occupational suicide". Putting it in writing would surely draw ire to any prospective employer not looking for it to begin with.
In my years working at one corporate employer, I saw as many as a half-dozen employees fired on the spot (for cause) when what they claimed in their initial job interview turned out to be untrue. Persons who just assumed no one would actually do the follow-up required. They were wrong.
Keep in mind while we discuss such things in this forum, that it remains an international forum. Where other members may offer a very different perspective given their nation and culture may well handle issues like autism in a much more favorable light than our own.