For high-functioning characters, I'd recommend a flat affect, little emotional engagement, formalized speech with polysyllabic words, non-mainstream interests, and an odd sense of humor. Make them tolerant of bright lights, crowds and loud noises for short periods of time. Otherwise, fully capable of interacting with others and taking care of themselves.
For medium-functioning characters, leave out the eye contact, make their selectively mute, make their movements seem as if the person perceives his or her environment as fragile or hazardous, and occasionally needing guidance in what to do in any given situation. They should be intolerant of crowds, bright lights, and loud noises. Routine becomes important.
For low-functioning characters, then occasionally catatonic, stimming, non-verbal, clumsy with a shuffling gait, intolerant of any sudden changes in his or her environment.