An audition is not a performance. It's an opportunity to get information about whether you will be able to get a good performance when the other elements of the film are in place. Information, not performance, is what the director should be concentrating on in casting. What you need to look for in casting is the actor's ability, whether he's right for the part, whether you and he can work well together, and finally, making sure you're casting not individual roles, but relationships. The actor's ability.
Ability is talent plus skill.
That is, the cards fate has dealt the actor through birth and experience added to whatever he has done to develop this natural talent. It's hard to separate which elements of an actor's work are due to talent and which to skill, and it doesn't matter anyway. If one doesn't develop his raw talent by learning skills, then the talent is not available. Central to an actor's ability is that, when he speaks dialogue, he is talking to someone, about something. A good actor is expressive in his face, eyes, voice and body, and is sensorially alive.