This was drawn in a restaurant and was a college couple at dinner. The girl, pictured in the first drawing, basically stayed in the same position. The guy, however, started out leaning back and then leaned forward and moved his body down until his chin rested on the table. He then started making circles with his right forefinger.
This was another conversation, taking place in a coffee shop. The guy moved his hands and arms so much that he literally "talked" with his hands.
This was a girl in Starbucks who was working on her computer. As I finished this drawing, she spotted me and stiffened up. Her entire body became straight, she uncrossed her legs and was very self conscious. After a little while, however, she relaxed and went right back into the same position shown here.
This was another girl in Starbucks. She stayed in this same position for a long time while talking to her friend. After awhile, she got up and went to the washroom. When she came back, she went right into the same position.
Body language is not only an important thing to understand, it's also fun. if you're interested in learning more, Tonya Reiman has a great book about it. Here's a little video of her talking about it: