You want the feel of authentic dialogue, but the spirit rather than the details. Listen to a cassette recording of yourself talking with friends. Embarrassment! Real life conversations are full of ‘um’s’ and ‘er’s’, hesitations, misunderstandings and unfinished lines of thought.
Okay, it’s good to include some of that noise for the sake of realistic flavour. But it would be maddening to have to read through the amount of it that goes on in real life.
Normally, we don’t even hear it, we hear through it, like ‘noise’ on any communication channel. We focus beyond the ‘um’s’ and ‘er’s’, can’t even remember them afterwards. Instead, we’re aware of what we intended to say, while at the same time reaching out for what the other person intended to say. Here’s the secret of authentic dialogue—in that mutual reaching out, that interactivity.
What I mean is that the spirit of dialogue isn’t just alternating statements, but alternating agendas and understandings. It’s a fencing match, with a common goal of mutual understanding, but also non-common goals such as asserting superior knowledge, withholding embarrassing facts, imposing a favourable perspective, forestalling criticism, etc., etc., etc.
Or like a couple of dogs sniffing each other out! We try to suss out the other person and what they’re really saying; and at the same time, we have our own things to say and our own preferred version of ourselves to communicate.
Convey those kinds of interactivity going on, and it hardly matters whether your characters (like Jane Austen’s) speak only in full sentences. Your dialogue is alive in the way that real dialogue is alive.
Most writers I know test their dialogue by reading it out aloud. I do. If it bounces along in varying tones, I know I’m getting it right. If it falls into a monotone—start again!