Note that this is just my observation, and how I try to describe this observation.
Pursuing body language and subtle stances as an autistic will be very tricky or very challenging.
Considering the common numbers of comorbidities, that includes the direct causes related to emotions and physical coordination.
Personally, my ace happened to be nonverbal aptitude.
My illusion, as a passing autistic, happened to be in kinesthetics than articulate verbosity that many aspies are proud of.
Reading and watching about theories of body languages aren't enough.
It's not even enough to mimic said body language through rehearsals like masking... Unless you have immense mental stamina -- you all know where this is going.
But it's something that can be projected, that includes subconscious.
The stance had to be drawn from intent and feelings translated into movement like an offering as a lure, not a repellant.
That includes repelling perceived would-be rejections based on one's preconceived worries, or attempting to repel your insecurities while trying to offer.
Not just to simply calculated mentally -- that will be still awkward or dwindling or dangerous as many here figured if one lacks enough resources and faculties.
Else, it's superficial -- just masking. Cheap attempt on performing illusions.
But how can an autistic do that, if they're all busy with the environmental chaos and confusion, obsessive thoughts and negative past experiences?
And how an autistic could figure if they cannot comprehend their own emotions and internal sensations, if they had no firm control over their bodies, if their processes between input and output is too slow, if they just had the great urge to stim everything away or the itch to move in certain ways?
That's how this so called 'social deficit' works. It's not simply just cluelessness or lack of social imagination -- that's just the one sided part that is 'trapped in their own heads'.
But also mechanical -- basically the 'trapped in their own bodies' kind of way.
From my point of view, I'm more trapped in my own body than trapped in my own head.
The latter mistakes my apathy in asocial nature, the social cluelessness and emotional immaturity that caused by executive dysfunctions like many.
And I'm not even the clumsy type to just claim such statements of being trapped in my own body!