Congruency is the word thown around. It is key.
If you act like a confident, strong man, then run like a little bitch in a confrontation, your image is shattered. Always be congruent. People will notice. It is the small subtleties of body language that give you away. I am self aware of it now. Sometimes when a super duper ultra hotty walks by me I might fidget a bit and wonder why I am fidgeting all of the sudden.
Watch Travolta... you can't find a spot where he's out of that confidenct character.
http://www.youtube.com/watch?v=_HKXqEwDKuc