It's the kind of pressure that can make even the most seasoned actor pooh his pants just a little bit.
Every Monday night I have an acting class in which I'm handed a scene and given a very short period of time to diagnose, memorize and then perform it in front of a camera. It's a technique known as "cold reading," and it happens often in real audition situations.
In addition to creating a rich history for my character and learning my lines, I have to tackle technicalities, such as: what if I'm supposed to be driving a car or laying in bed? Or talking to three different characters even though only one casting director will be in the room with me? Are my hands gesturing too much, is my voice too soft, are my eyes down on my script too long, etc., etc., etc...
Now imagine corralling all of this in front of a room full of producers from a hit TV show. Or entering and finding out that Denzel has decided he's going to be there reading his role opposite you.
Pressure. Nature's laxative.