They cannot spend hours going over and over the scenes, correcting an actor’s choice repeatedly if they fall short on their execution. They cannot keep cutting if an actor does not know the proper way to present their body toward a camera or if the actor keeps stepping out of frame. Luckily most movies are shot on video these days rather than actual film, which is quite costly. Some do still shoot on film, however, and film cannot be recorded over. If an actor needs on screen coaching throughout the scenes and the director has to keep cutting, that film is wasted—that money is wasted.
