Since we see everything through Jake’s perspective, it’s pretty easy to figure out his opinions on the other characters. He is very up front with us about everyone, and often tells us directly (and sometimes in great detail) what he knows about his friends. A good example is Robert Cohn: from page one, we are immersed in a lengthy description of Cohn’s characteristics, good and bad. Jake is careful to at least try and give us a fair and objective portrait of Cohn in these first pages; once, he even stops himself ("Somehow I feel I have not shown Robert Cohn clearly") and starts over again to get the facts right. In these early chapters, we come away with a picture of a harmlessly irritating guy whose main flaw is that he’s kind of a dupe. Even though Jake can’t entirely contain his disdain for Cohn’s weakness, we know that he likes Cohn despite his flaws (something that will change soon enough).
As a narrator, Jake is very into showing us exactly what happens in the course of the book – if he has a meal, he tells us exactly what it is, and, if he has a conversation, it’s recounted faithfully in the text. The same is true of significant actions taken by the characters. The perfect example is Pedro Romero’s performances in the bull-fights. Romero’s fighting style reflects upon the purity of his character, and, to help us understand what the young matador is like, Hemingway describes every movement he makes in the ring with incredible detail.
Another character whose actions, though not always shown, comment upon her personality is Brett. She comments constantly about her need to bathe, often in the imperative ("Must bathe"), which reveals to us her hidden fear that she’s unclean and needs to be purged of her sins.
The convoluted network of sexual relationships of the characters plays into our understanding of them. First of all, Jake, who is excluded from sexual relationships, dwells upon the affairs that Brett embarks upon, and his bitterness increases because of his inability to be with her. Brett’s various liaisons with Cohn, Mike, and Romero all create conflict between those characters, and the men in her life all demonstrate their jealousy in different ways. Their ways of dealing with their unhappiness reflect their various backgrounds and personalities. Mike, for example, throws himself into an even deeper pit of drunken bad behavior, and refuses to take responsibility for himself, while Cohn, clinging to his outdated notions of chivalrous love, retreats into juvenile violence.