Salinger is a real master of voice and dialogue, which he puts to good use in "For Esmé." We really get to know Esmé and Charles through their conversations with the narrator; in fact, that's pretty much all we really get about them, except for the odd observation about their looks here and there. Salinger does an amazing job of creating hilariously lifelike voices for both of these kids; Esmé's too-mature, self-consciously self-important tone makes her a believable thirteen-year-old, who's balancing between childhood and early adulthood, while Charles' hyperactive voice brings to mind every five-year-old we've known.
The conversation between Clay and Sergeant X is also incredibly important, and reveals a ton about what's happened in the time between Devon and Gaufurt. We can clearly see the change in Sergeant X, who's no longer the innocent, youthful guy we saw in the first part, and his cold, sarcastic interaction with Clay, who's supposedly his friend, is also telling.
We get direct access to the thoughts and opinions of the narrator/Sergeant X, and they reveal more about him than anything else. In the beginning, we can tell from his inner observations just how much he enjoys talking to the kids, and we get a privileged view into his reactions to the various hilarious things they say. In the second half of the story, we still maintain our direct feed into the character-formerly-known-as-the-narrator's mind – and it's a very different mind, indeed. Sergeant X, as we're told, "is a young man who did not come through the war with all his faculties intact" (106), and we see right into his unstable, unhappy state of mind.
The narrator (whether first person or third person) really cares not only about the way people act and speak in this story, but also about the way they look. We get descriptions of Esmé, Charles, and Clay, and all of them are somewhat defined by their physical appearances. Esmé, for example, is an unusually beautiful child, whose physical grace and killer sense of style mark her as special. Her "blasé" eyes are also a point of interest – they seem older than she is somehow, and demonstrate her unique maturity and perceptiveness. Charles is also a handsome kid, a fact that the narrator emphasizes – because he's adorable, his erratic behavior is charming, not irritating. Finally, Clay is a big, beefy, phlegmatic guy, both outside and in; there's not very much lurking beneath his shiny, happy exterior. Sergeant X, on the other hand, is thin, unhealthy, and has a facial tic – his mental distress is directly reflected in his physical state.
This isn't a huge, huge deal here, but one of the ways we get to know Esmé is through her own very self-conscious understanding of her own social status: she's an aristocrat, and clearly isn't quite sure what to make of it. This uncertainty is exhibited in her refusal to tell the narrator her full name, just in case he's intimidated. Her assumption (perhaps correct) is that Americans, who exist outside the British class system, can't really comprehend what her official social standing means, and therefore are uncomfortable with it.
This is an easy one – the narrator/Sergeant X is a writer, and this knowledge of his occupation helps us understand his perspective, and his desire to tell his story truthfully (and elegantly!). It also encourages us to see him as a perceptive and observant young man, and perhaps explains his "sensitive" nature.