Body Language.
I've been studying it for years now (with major thanks to the FBI guys at my uni, lol) -- NT's pick up immediately on leg positions, what you're doing with your hands, feet, shoulders, legs and head, not just your facial expressions.
Just doing something as comfortable as putting your hands behind your back or crossing your ankles can give the message of "leave me alone" or, with the locked ankles, more of a "I'm feeling defensive already, go away".
If you step away to let them through a doorway, depending on the position of your body, will give off a signal that yes, you're getting out of their way, but you're not actually happy about it.