Originally Posted by
Melange
What you do pay attention to most commonly is, in my experience, four things, assuming it's not aggressive conversation; their gestures, how they're talking, what they're saying, and where they're looking. The really tricky part is creating interaction between characters who know each other really well; they make references, leaps of logic and crack inside jokes that don't immediately make sense to the reader. It's something I tentatively labeled "atmospheric communication"; they just know what the other party is thinking and can react accordingly, but the reader usually doesn't know what either party is thinking and has to connect the dots themselves. Thus the author has to add in enough hints for the reader to guess accurately from, but not enough that the interaction seems broken or out of place. I tried to craft most of Zeph and Whimsy's interactions in that way, where you can guess what they're thinking, but it's obvious they're on the same wavelength and understand each other. Those are the trickiest types of character interactions, as far as I'm concerned.
(In essence, atmospheric communication is like when you're sitting in a room with a bunch of people and something weird happens, like a door is kicked down, and you by chance make eye contact with someone else, and with no other communication both of you instantly know exactly what the other one is thinking.)