We learn several things in this act about the characters. It is the climax or turning point of the story, so the characters are at their highest stress levels and show their true characteristics. The first is Tybalt. We knew he had a fiery temper all along, but in this scene he went as far as killing (whether accident or not) Mercutio. Then we see two different sides to Romeo. He shows his tragic flaw in the first scene--his decisions are never thought through completely. He acts rashly. He jumps before thinking and ends up killing Tybalt out of revenge. Next he falls apart completely and almost takes his life in the Friar's cell because of his banishment.
Next we see a different side to the Nurse. When it comes down to it, she is nothing but a servant for the Capulet household. She must do as she is asked. She can't help Juliet deceive her parents any more. She tells Juliet (as she is making up the bed that Juliet and Romeo recently shared together) that she should marry Paris after all. Juliet doesn't like this at all. She answers the Nurse with "Amen." That single word means "that's the end." She wants no more to be involved with the Nurse if she isn't going to support her love for Romeo. The Nurse is in a hard place, but must do as her master tells her. Lord Capulet yelled at her, so she has to follow him and what he tells her to do.