Some call it characterization.

There are two ways that Hemingway develops characters in the novel: direct characterization and indirect characterization. Most direct characterization is done with Cohn. Hemingway needs to make sure that the reader understands Cohn and his character traits, so he tells the reader things like "He was a nice boy, a friendly boy, and very shy, and it made him bitter," (page 12). This type of characterization gives the reader very clear descriptions of the character that show their development and personalities in a very easy to understand way. The reader now understands why Cohn does what he does, because they are told that he is bitter and what made him that way.
The other type of characterization is one that must be understood by the reader. It can be seen in the ways that the characters interact. Mike makes fun of Cohn and insults him for hanging around Brett. This tells the reader that Mike is very insecure. Also, Brett chooses Jake to talk to with her problems, showing the reader that Jake is a good listener and kind. Hemingway is able to show the reader what the character is like instead of just telling them.
The effects of these to types are that simple, strong character traits can be established through the direct method. This ensures that the larger, more important traits are understood by the reader. The more complex character traits and character developments can be shown to the reader through interactions with others, creating a better picture than the first method. Both are used by Hemingway, but their intentions and results are slightly different.