In this scene I would want an open window clearly visible in the background. When Macbeth says “Methought I heard a voice cry ‘Sleep no more!…” the curtains covering the window should begin to blow gently. They should speed up and become more aggressive until Lady Macbeth leaves. Just before Macbeth hears the knocking, the window will slam shut on its own. These elements will begin to display how frightened Macbeth is.