Answer:
Although you have not shown which play you are referring to, we can state that the stage instructions help the reader to understand what is happening, because they show how the character behaves in relation to his personality and posture, in a given situation. Furthermore, when it is stated that the "author is in the shadows" implies that although he has a certain influence, the author is not actively participating in the scene.
Step-by-step explanation:
Stage instructions are, as the name implies, instructions that the author of a play establishes to show the actors how the character should behave and move in a given situation. The purpose of these instructions is to let the actors know how the message should be conveyed to the audience through the character's movement, facial expression and behavior. These instructions also serve to help readers understand the situation in which the characters are inserted and how it affects them.
When the statement "the author is in the shadows" is provided, it implies that the author is not actually participating in a situation, although he may have some influence.