The fourth and most important difference between a novel and a film (or play) is that when the reader tires of a novel he can mark his place, put it down, and return to it later. But the attention of an audience must be held continuously. There must be an unbroken progression. It may be progression of the emotion or the thought or the action, but emotion and thought must issue in action or threaten to. In a dramatic medium such as film the characters cannot pause to propound ideas and emotions not directly relevant to their own dramatic situation. In the middle of War and Peace Tolstoy can plant a substantial essay on the nature of military power. In a film script one unnecessary page, one page not furthering the progression, will lose the attention of the audience for the next ten.

Robert Bolt (1924-1995) English dramatist
Doctor Zhivago: The Screenplay, “Author’s Note” (1965)
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