When you are standing in front of the Casting Director right before your audition begins, this is the make it or break it point. If your thoughts are about "I want to impress this Casting Director" or "I hope I do a great audition" then the odds are that your audition will get off to a rocky start. You won't be warmed up until half way through the page and by that time you've lost the Casting Director. But, if your thoughts are of the scene itself, what happened right before the scene starts... specifically...then you will begin the scene in the middle of something.
I always liken the actor to the athlete. The mental focus the athlete must have is to focus on the task at hand. "The tyranny of the scattered mind", as discussed by sports psychologist H.A. Dorfman in his book "The Mental ABC's of Pitching", can be the downfall of the pitcher. If the pitcher is thinking a thousand and one things other than his task, he needs to step off the rubber. Dorfman writes that the pitcher should be thinking about 3 things: Pitch selection, pitch location and the catcher's glove. The pitcher needs to focus on simple tasks that will help him control his scattered thoughts. Thus, achieving mental discipline.
The mental focus the actor must have in the audition room should be on simple tools to control their scattered thoughts. If the actor is thinking about wanting the job, impressing the Casting Director or believing another actor in the lobby is more right for the part, "the tyranny of the scattered mind" will sabotage their mental focus. They will not be focused on the scene and they will not be doing their job as an actor.
One of the simple tools used to control mental focus in the audition room is "the moment before". Be specific. Remember the scene starts in the middle of something. If you don't know from the script what just happened, you have to make something up for yourself. What was just said, what just happened, visualize it and...go.