I find the dialogue in many novels to be quite unconvincing. People — at least for the most part — do not speak in these long-winded paragraphs of which some writers seem overly fond (Dostoevsky!) but are rather inclined to short fragments of speech, with plenty of pauses. Realistic dialogue should reflect this fact.
Now, realistic dialogue may not always be the aim. My fantasies tend to go with more formal speech, at least for the upper class individuals. Even so, I would rarely make a statement more than a couple sentences long. There is very little speech-giving in the course of conversations.
Television writers seem to recognize all this. They strive to write short and tight dialogues for their characters. No, it does not always come off as believable (especially when an info-dump is spliced in) but it is certainly more realistic than what is found in many novels.
Humans do tend to be incoherent. Accept that fact. They rarely have ‘talking points’ planned out before they open their mouths. They misunderstand each other. Yes, one of the goals of the fiction writer should be to make it all more comprehensible, but not at the expense of making it artificial.
So don’t let your characters blab on without taking a breath. It is unrealistic and it is boring. Let them pause and consider. Let facial expressions and body language be noted. A conversation is more than just words, after all. It is two (or more) people interacting in a variety of ways.