I never mean to exactly, but I often grin like a maniac when talking to strangers, though the effect goes with people I am >80% comfortable with, which is basically limited to my mother and elder sister. The smiling feels like instinctive fakery, but it has been called out in the past by my psychologist, who has some professionally awesome people-reading capabilities, and more recently by the specialist who diagnosed me with ASD.
Regarding the latter, she had just made her final statement after taking some time away from me and my parents (trapped there for 5 hours!) and looked like she expected me to say or do something different, but I just sat there with that stupid grin plastered across my face, despite being rather overwhelmed and upset.
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On hiatus thanks to someone in real life breaching my privacy here, without my permission! May be back one day. +tips hat+