There's a famous story about James Gandolfini- the actor that played Tony Soprano. On the day that he walked into a casting room to audition for series creator David Chase, Gandolfini read through his lines, but something didn't feel quite right. He was going through the motions, and wasn't quite connected with this character. David Chase, along with the casting directors in the casting room also felt the same way and the energy was lack luster during Gandolfini's audition.
Then something interesting happened: Gandolfini, at the end of his audition, admitted to blowing his audition, and then asked Chase if he could back the following day to read again. Chase- appreciative of Gandolfini's honesty- agreed, and, well, the rest is history. Gandolfini went on to play Tony Soprano on "The Sopranos" for 6 seasons, and became one of the most famous and well-regarded actors of his generation.
But what is fate had not intervened that day in the casting room, when Chase and Gandolfini were both collectively disappointed with Gandolfini's audition? The point is that as casting directors and filmmakers, we never quite know the potential that an actor possesses. Sure, if an actor completely blows their audition, then it's probably safe to say that they probably don't have what it takes to play a particular role, but if someone's performance is slightly off, and yet you see a spark of potential, it's always good to err on the side of graciousness and believe in the art of what's possibly.
Consequently, casting directors shouldn't think of casting sessions as rigid, infallible environments where they hold court over a nervous group of actors. Rather, they should think of their casting rooms as places where artists are allowed to explore, make mistakes, experiment, and ultimately land on the heart of a character. In other words, sometimes casting directors find perfection- but most of the time, their task is to nurture it.