The realistic elements of A Streetcar Named Desire are mostly based on the cruelty of Stanley Kowalski, the sexual co-dependence of Stella, and the wreck of life that is Blanche Dubois.
Scene one places Blanche asking for her sister's address in a raggedy, dusty, and evidently working class New Orleans street. Blanche's white and elegant dress contrasts enormously with the sounds of everyday life and the dump of a place where is standing.
Realism here is obvious when we see that Stella has moved from a former rich life to a dirty, small, and stuffy apartment with only one bathroom, two bedrooms and no doors. Already this appeals to the senses of the audience as one feels the uncomfortable situation in which Blanche enters.
On Scene I we also learn about how Belle Reeve was lost to creditors, how Blance claims to have stayed to save their mansion, and the pain she had to endure with the death of her parents. All of these situations are realistic in nature and are not "sugar coated" with figurative language and symbolism. That is what makes naturalism and realism different from other genres.
Scene two of A Streetcar Named Desire starts to reveal the mysoginist nature of Stanley's character. He is rough, miserable, and brings the whole atmosphere of the house down. He is disrespectful to a point (he will be worse later), and he is clear in that he is willing and able to hurt Stella, Blanche, or any woman that crosses him. In this scene we see again the roughness of nature at its best. We sense the danger, we resent Stanley, and we fear for the women's lives. Once again, there is no sugar coating on this one. The girls are on their own, and at the mercy of a man capable of all the evils in the world.