As I understand, when someone signs, there are three parts to the "sign" for a word:
1. A facial expression (to denote if this is a joke, sarcasm, or fact)
2. The hand gesture
3. A mouth movement to denote the word that is being signed.
There is a lady in church -- the guy I am dating is a Lutheran preacher -- who signs for her deaf mother, and I've learned so much just by watching her...I have no idea what the words are to the Lutheran litergy song thing, but I can damn near sign the entire thing...
I also remember when I traveled abroad in Europe. It was amazing that in a couple of days, I could figure out what everyone was saying around me just by watching them. I didn't have to know the language because there was so much conveyed in body language that I could get the jist of what people were saying...