What role does Stella play in the conflict between Blanche DuBois and Stanley Kowalsky?

What role does Stella play in the conflict between Blanche DuBois and Stanley Kowalsky? What, for example, is the significance of the scene in which Blanche tries to convince Stella to leave Stanley? What is the full significance of Stella’s line, in scene 11, that she couldn’t believe Blanche’s story "and go on living with Stanley"?
The only source required is the Play "A Streetcar named Desire" by Tennessee Williams ( this play is available also as movie with Marlon Brando)

Please only US writers,