When you ask for changes, make your points clearly and directly. Sadly, Hollywood agents have made rehearsals for Hollywood movies an expensive commodity, but with unpaid actors and with actors who want to help the production, better arrangements can be worked out. No director should scream in anger at his cast. He does himself and them a disservice. Calmness and patience work much better.
Actors sometimes give less than their best performances because they feel under pressure.
When Richard Bare told an actor after 17 bad takes that they had plenty of film and plenty of time, the actor got much better. It can be very difficult for a young director to tell an experienced actor that his performance is no good. Then of course, the director must allow the actor to develop his own performance within the context of the directors overall view of how the film should develop. The deepest principle in human nature is the craving to be appreciated. Actors respond well to the director who tells them when he is pleased.