At the core of your story are your characters. At the core of your characters are their hopes, dreams, fears, sorrows, shames, etc. that drive them to action and, in turn, drive your story.
Actors are amazing. They willingly thrust themselves into the persona and life experiences of another character. We, directors, are constantly challenging them, provoking them, and hopefully inspiring them to fully embody the emotional states of these characters.
Yet, what many directors overlook, or are not aware of, is that there is an evolutionary change in every actor as they move through the key phases of filmmaking: Auditions, Rehearsal and Production.