When Blanche first appears on the scene in New Orleans, Williams uses visual imagery that aligns her with a pale, delicate insect. He does so to portray her as both out of place and fragile within the gritty environment of the French Quarter. The stage directions explain:
[Her delicate beauty must avoid a strong light. There is something about her uncertain manner, as well as her white clothes, that suggests a moth [...] ]
The stage directions here are intended to give the director or reader of the play an idea of how the author envisioned the scene playing out. Williams describes Blanche’s delicate beauty and her need to avoid strong light at this early point because they are two of the most important things about her. He accompanies this description with a note about her attire, noting her preference for white clothes. In a city where (as the other stage directions make clear) there’s a lot of urban and industrial grime, Blanche’s white outfit casts her in an ethereal and almost otherworldly light.
The choice of white for Blanche’s clothing is important symbolically as well as literally. The color white is regularly aligned with ideals of purity and innocence. It’s also very easy to mark or stain, so writers often use it to denote fragility or delicacy. In Blanche's case, her white attire is part of her futile gestures to presenting herself as untouched and virginal. This portrayal is, of course, totally at odds with her actual situation and her past troubles This visual imagery suggests—as Williams says explicitly here—that she resembles a moth. She’s not a butterfly, but something more modest and delicate. Like a moth, she’s also inherently and self-destructively drawn to light despite its dangers.
After the harrowing scene where Stanley drunkenly beats Stella during the poker game, he has an abrupt emotional turnaround when he realizes Stella may leave him. Williams makes sure the importance of this moment is clear using auditory imagery in his stage directions and in Stanley's words (or, rather, word). Standing outside their apartment, Stanley screams for his wife:
STANLEY [with heaven-splitting violence]:
STELL-LAHHHHH!
One of the most iconic moments in A Streetcar Named Desire is this auditory imagery of Stanley standing outside his apartment, shouting for Stella with "heaven-splitting violence." It's the scene the play is best known for—particularly Marlon Brando's muscular delivery of it in the 1951 film adaptation of the play.
Although it seems straightforward, this scene is very important for understanding both Stanley's character and the dynamics of his relationship with Stella. This yelling happens after Stanley viciously and drunkenly beats Stella, taking his frustration (with Blanche and with poker) out on his wife. In this scene, he's standing outside their apartment screaming her name, trying to make her speak to him after he's been forcibly sobered up. It's notable here that even when he regrets his violence toward his wife, he’s still acting “violently.” He can’t use words to apologize but instead has to “split the heavens” with a primal, animalistic scream.
Stanley's scream is also a plea, a moment of remorse. Stanley is an aggressive and brutal person, but his relationship with Stella is almost entirely based on physical acts. They express their desire for each other through sex, and they express their anger with each other through violence. Here, Stanley is terrified Stella will leave him, and so he appeals to her in the only way he knows how.
When Williams first describes Stanley and Stella's apartment during the poker game in Scene 3, the stage directions evoke the visual imagery of primary colors and unrefined, direct light for the play's reader or director:
The kitchen now suggests that sort of lurid nocturnal brilliance, the raw colors of childhood's spectrum.
Williams’s stage directions are famously poetic and evocative, bringing an air of the literary to more than just the speech of his characters. The description of Stanley and Stella's apartment here makes a point of its vivid, childlike colors. Unlike Blanche, who hides in the shadows and avoids strong light at all costs, the Kowalskis thrive in it. Indeed, when Stanley and Stella have sex, they refer to it as “getting the colored lights going,” which reflects the openness with which they treat physical acts. Stella has no reason to hide her face or her body—sex, for her, is compatible with light.
The term "nocturnal brilliance" suggests that the apartment is penetratingly bright. It illuminates both the New Orleans night and Blanche’s place in it. Before Blanche arrives, the French Quarter apartment is perpetually illuminated with “the raw colors of childhood’s spectrum.” For Blanche, who’s well past childhood and who arrives with a penchant for dimness to obscure her reality, this constant brightness poses a threat. While Stanley and Stella live in a rough but honest state of mutual sexual desire, Stella is only able to attract the same desire in the shadows. The ideas that the lights in the kitchen are “raw colors” associated with "childhood's spectrum" further emphasizes a sense of straightforwardness and simplicity. This contrasts sharply with Blanche's muted, moth-like appearance. Blanche and Stella are two opposite ends of the same spectrum: Stella is all childish openness, and Blanche is all adult complexity.