Smile large with wide open eyes. Then suddenly look frightened, turn your head to the side and mumble quickly. Stare at the person/people paranoidly, then laugh nervously and start to tug at your ear or your hair lightly.
You have to keep this up, keep looking happy... overly happy. Like you are covering something up. You need to mumble when you *think that no one is looking*, even if you know they are. Chuckle, snigger to yourself. Like an evil mastermind, then suddenly snap and start shaking your head. "No. Hurting is bad... don't want to hurt... no. Stop it..."
Then, you stop with all that. You stop for a while. Then you're acting normal, happy normal happy happy... just suddenly snap into anger. Cover your ears with your hands, double over and scream at yourself to shut up. Look from side to side dramatically, scared, paranoid. Back away, stumble, like you are about to be eaten alive by invisible bugs. You ARE going to be eaten alive by invisible bugs! Make yourself believe it! Then, sink to the ground after screaming and whimpering and begging for them to "leave [you] alone". Curl up, sobbing but slowly... slowly turn the sobbing into laughter. Maniac laughter. Constant CONSTANT LAUGHTER. HIGH-PITCHED, WHEEZY LAUGHTER. Then stop.
And stare out blankly.