I’ve heard from a number of Casting Directors recently, that they know whether you have a shot at the part as soon as you walk in the door. Bonnie Gillespie (imdb, blog) says that “your energy walks into the room before you do.” Effectively, your first impression may very well be the deciding factor in your audition success. Not your reading. Not your preparation. The way you walk in the room.
Basically, we’re talking about first impressions here. Like it or not, they can be extremely effective in judging things like extroversion and self-esteem (source), the latter being a crucial factor when casting someone. Confidence is absolutely critical in auditioning, and if you have it, you will make a great first impression.
Homework: Start studying first impressions. See what your first impressions are of the next 5 people you meet. Why did you have those? If you got to know that person a little better, do you think your first impression was accurate? Start asking people (anyone: close friends, recent acquaintances, whomever) what their first impression of you was. How might you improve that?
Report back your findings. =)