In a story about beautiful, charming, young socialites, you expect your characters to be, well, beautiful. And they are. We don't get a ton of description of our characters, but we know that both Bernice and Marjorie are quite lovely. Notably, they're polar opposites – Bernice is dark and Marjorie is light (that is to say, Bernice is a brunette and Marjorie is a blonde). This is also tied loosely to Bernice's difference; her dark hair and rich coloring remind us subtly of the fact that she has "crazy Indian blood" (34), while Marjorie, the stereotypical picture of conventional beauty, looks like "a delicate painting of some Saxon princess" (121).
Snappy speech and slangy dialogue are characteristic of Fitzgerald's characters across the board, particularly in his commercially successful short stories. It's particularly important here – both to us, as discriminating readers, and to Bernice herself. What Marjorie teaches her cousin isn't all about outer beauty and style – it's mostly about how to converse and entertain. The kind of conversations we witness do a lot of the work in defining the modern woman to us – we get to hear Marjorie's vehement opinion about what a girl needs to do to keep men interested, and we also see these demands in action through Bernice's post-makeover interactions.
Speech and dialogue also show us a great deal about the relationships we encounter here, as well as the power dynamic between Bernice and Marjorie. The latter is totally dominant when it comes to talk – Marjorie's quick wit, stinging repartee, and rather vicious directness bowl over her tongue-tied cousin. However, Bernice ultimately wins when it comes to action.
The narrative voice in this story doesn't interfere much to place judgment – we only very occasionally hear it offer some direct characterization, and even then, it's rarely of the main characters. Instead, Fitzgerald lets us in on the thoughts, opinions, and judgments of the characters themselves – instead of telling us right out that Marjorie is a rather misogynistic snob and a half, we learn that "she considered girls stupid" (25). Likewise, we learn a lot about Bernice through her thoughts and feelings – she "felt a vague pain that she was not at present engaged in being popular" (27), a revelation that alerts us to the fact that Bernice isn't used to her own unpopularity, but she's not sure what its cause is.