Realism was a literary period which came about in the mid-nineteenth century. Realism depicted life as it really was, using common settings and characters. Not completely happy with the genre of Realism, Naturalists emerged in order to illuminate, and change, specific ideas within Realism.
Naturalists used common settings and characters in the same way Realists did. Naturalists also took an objective stand when it came to the point-of-view. Both Realists and Naturalists did not want to "interfere" with the story being told. Instead, "their" narrators mirrored scientists (in regards to how they simply recorded the "facts" as they happened). The narrator was not allowed to interfere with the story.
Perhaps the most important characteristic of Naturalism is the giving of power to nature. For the Naturalist, nature was more powerful than anything else, mankind included.
In regards to Tennessee Williams' play A Streetcar Named Desire, one could define specific characteristics which are Naturalistic in nature.
First, the setting is very realistic and common. The play takes place in Elysian Fields, a neighborhood in New Orleans. The setting is described exactly as one would expect to find in a rather poor, working-class section of the city. The music is also typical of the time and place.
Second, the characters are realistic and common. Stanley is the typical working-class man's man. He swears, drinks, bowls, and openly abuses his wife (which is accepted and forgiven by Stella). Blanche is the epitome of the broken and aging Southern Belle. Essentially, the play is filled with stock characters.
Lastly, no matter what any of the characters do, nature "wins." Blanche, in order to hide her aging, covers light with paper lanterns. In the end, Mitch removes the lantern and sees that she is older than she has stated (showing that man cannot cover up the natural aging process). Also, Stanley's brutish nature is far more powerful than anything else. His rape of Blanche shows the natural power of man over woman. Lastly, Blanche's diminishing mental nature cannot be stopped. In the opening, readers can see that Blanche is "not quite right." Her behavior and dialogue is confusing at times, and she tends to believe in alternate realities. In the end, the nature of her brain's weakness cannot be stopped.