When J.D. Salinger’s characters are talking, they are oblivious to the rest of the universe. They don’t deposit buckets of information. They don’t talk about yesterday or tomorrow (unless they’re fighting of course). They have their own emotional agendas and that’s it.
During a real fine piece of dialogue, the middle man (the author) disappears. Just like in Hollywood, the viewer never sees the camera - that would kill the whole illusion - the viewer actually believes that he or she is actually there, in the action, eavesdropping, watching.
When I read Hemingway I have to work real hard to find out what his characters are talking about. They don't gift wrap the conversation. They are selfish, and don't give a damn about the reader.
In a good piece of writing, the author and the reader BOTH become invisible, and all that's left is the story.