I think that it is in the Fourth Scene that the nature of the main characters begins to emerge.  On one hand, Stanley's vindictiveness is on large display.  He understands Blanche as a threat, and rather than seek to understand "the other," Stanley simply wants to obliterate it.  In this light, Stanley represents a dominant force that seeks to crush all other opposing elements in his path.  His desire to want to find out more about Blanche is expressed in a manner that discredits her, and hastens her departure.  Blanche's insistence that Stella leave Stanley is done out of a different motivation.  Blanche's dependence and her absolute terror of being alone is present in her pleas to her sister.  She does care about Stella's well- being, but in pleading with her to leave, Blanche is making a desperate stab at finding someone, anyone, who could take care of her and nurture her.  Blanche's emotional nudity is on display for all to see, something that Stanley understands and exploits.  Stanley's primal instincts to protect what it his and eliminate any threats from the outside is what ends up defining him throughout the play and undermining Blanche at the same time.