Ernest Hemingway's The Sun Also Rises is written using a first person narrator. First person narrators play a role in the action of the story, act as the protagonist (main character), and tend to tell the story of his or her life (and the conflicts he or she faces).
Very different from the elaborate dialogue and sentences in The Great Gatsby, Hemingway uses short and simplistic sentences to push along the fast pace of the novel. The dialogue tends to be realistic in nature (readers can see people actually having the conversations the characters in the text have-given the lack of constantly elevated language). Hemingway's characters speak (or so it seems) realistically and truthfully (to human nature, not to the truth itself).
The structure of the novel mirrors the technique Hemingway applies. The sentences tend to be short, choppy, and fast moving. The simplicity of the sentences and word choice are determined.
Like most authors, Hemingway also themes and motifs to speak to an internal or hidden message. Structurally, the themes come out in the realistic actions and dialogue (morality and meaning in life). IN the same way, the motifs (communication, drinking, and friendship) need to be "persuaded" out the terse dialogue--readers need to "read into" the text.