I've always wondered. The sign languages I am familiar with have specific codes/rules for their body/facial languages (the technical term is non-manual markers. Stuff you do in the language without using your hands. Like nonverbal communication in English) for example, furrowing your brow is the way you turn some verbs from actions into questions (only like a couple hundred people use the sign language I use to my knowledge so I'm going to talk very vaguely about ASL or SEE. I've only seen one person outside of my little group use it and that's Carel Struycken, who you might recognize as playing The Giant on Twin Peaks. I would love to speak to him someday and find out how my group and he learned the same sign language.