In Williams' "A Streetcar Named Desire", the tone is very important.
In the play, there remains one major tone that follows the actions of the play throughout. Depicted by the lone blue piano, the tone of the play shows as one which mirrors that of the characters- depressed, solitude, and violent.
Specifically, in Scene Four, the tone is set as anxious. In the previous scene, Stanley has attacked Stella and Stella left their flat. Blanche and Stella are talking about the night before and Blanche cannot understand how Stella can stay with a husband who beats her. Readers, depending on their own background, will either side with Stella or Blanche.
To explain, only people who have lived under certain circumstances can fully understand the decisions which go with them. Therefore, women, or men, in abusive relationships may have the same emotional attachment as Stella has to their abuser. As for Blanche, she cannot understand how Stella can stay given she has never been in the situation.
This being said, the tone of this scene can be different for each reader. One who has lived within an abusive relationship may find understanding and hope in the words Stella speaks, whereas others, who side with Blanche, may feel angered at the fact she chooses to stay. The tone from Scene Four changes dramatically after the action of the previous scene.
As for author intent, it could be rationalized that Williams is simply stating the "facts" in a Naturalistic fashion. He is showing both sides of how different characters would/could react to such a situation as described in Scene Three.