[llevrok]

psychopaths and sociopaths are largely narcissists.

Keeping their heads “quiet” as they speak may be their subconscious saying “look at me!!!”

[piasa]

Could just be muscle tension caused by the difficulty of thinking up plausible lies.

[Rockingham]

Or their reptile predator brain is dominant, which keeps the head still in order to be ready to pounce. As much as that sounds like a just so story, there are MRI scans that indicate that in psychopaths the empathetic areas of the brain are damaged due to lesions, with the brain adapting by relying more on the older reptilian brain for high level decision-making.