Colin Barrett, interviewed by the Paris Review (full text here) talks about what makes good dialogue:
I look at dialogue spatially, in terms of its placement on the page, the way it breaks up the otherwise solid blocks of text. The reader, in the corner of their eye, can see it coming, can anticipate a shift in rhythm and pacing, the speed at which their eye will move laterally and vertically along the page. In terms of making dialogue “good,” there has to be a rhythm, some sort of cadence. Readers will buy ornate or otiose phrasing if it’s biding by the flow in which the speaker talks. As a writer, don’t be stingy. Give the characters good lines, give them the best. Every so often you come across writers who give the omniscient narrator or diegetic voice these rich powers of articulation, but the characters all talk like dummies.