Yes, that’s right—the early interactions between Stella and her husband constitute the initial situation of A Streetcar Named Desire. It’s important for us as the reader/audience to see the status quo of the Kowalski's relationship before Blanche shows up and alters it for the duration of the play.
Blanche arrives; something is up.
The immediate physical incongruity of Blanche and her surroundings lets us know that she isn’t going to fit in well here in New Orleans. Her first conversation with Stella hints at secrets she’s trying to hide. And her first encounter with Stanley is wrought with tension, sexual and otherwise. All the news of the loss of Belle Reve doesn’t help, either.
Blanche’s relationship with Stanley grows more and more antagonistic, especially as Stanley learns more about Blanche's past in Laurel.
Blanche and Stanley's relationship grows more and more difficult, with Blanche constantly insulting him, and Stanley becoming more angry and aggressive. Stanley also learns about Blanche's secret past, which he informs Stella and Mitch of.
These multiple, small complications are what modern writer and essayist John Barth calls "incremental perturbations"—the water gets muddier bit by bit as the play progresses, and every new complication adds a layer of intensity and emotional weight to the story.
Scene Ten—the rape.
Did you notice that Stanley says to Blanche, "We’ve had this date with each other from the beginning!"? We know that 1) Stanley doesn’t like Blanche, 2) he takes out his anger physically, and 3) he’s practically defined by his sexual aggression. This scene seems the inevitable result of their increasingly antagonistic relationship.
Too late—already happened.
In this play, the suspense stage can be found in Scene Ten with the Climax. The suspense builds as we watch Blanche interact with Stanley, make a frantic phone call, declare repeatedly that she’s "caught in a trap," and try to run away. Once the rape is over, we enter Scene Eleven without further suspense.
With the rape and the birth of Stella and Stanley's child over and done with, the play’s final scene has "falling action" written all over it. Blanche’s descent into madness is complete, and we’re now looking at the aftermath to the destruction that took place at the earlier climax.
Stanley and Stella on the porch together
Stella’s reaction to Blanche’s condition and story regarding her husband, and her decision to carry on her marriage in spite of it, constitute the play’s conclusion. This is summed up nicely in the image of her sitting on the porch with her baby in her arms, accepting comfort from her husband after her sister’s just been carted off to an institution.