"If you can't fully believe in your ideas, it very quickly communicates to a group of actors who need something to hold onto. They need to believe that whatever criticism, whatever comment is received, is meant"

This quote by Trevor Nunn talks to the value of believing in your concepts when dealing with a group of actors. If you do not believe your concepts, it will quickly be communicated to the actors, who need something to latch onto in order to perform their finest. Without this belief, any criticism or remark gotten by the stars will be taken as a lack of confidence in their work, which can be damaging to their performance. Nunn is stressing the importance of having faith in your concepts and conveying that to the actors, so that they can trust that any feedback they get is implied to help them improve their performance.