Each character should have aspects of dialogue which are theirs alone among the other characters. This can be as obvious as using a personal ‘catch phrase’ to the use of double negatives. They can constantly use humour, or inappropriate humour, or be self-important. They can use too many pauses (…). The author is careful not to overuse these devices – perhaps only once in a chapter. Dialogue, in addition to relaying plot information, is also a character-developing trait.
Dialogue in a novel is not “real” dialogue. What I mean by “real” dialogue is dialogue spoken by people in conversations, using air and voice. We do not just put down verbatim (real) conversations. They do not work on the page. Written dialogue is *concentrated* (much like juice) dialogue, doing much more than just responding to sentences. Each line does much heavy lifting in the progress of the novel.