Dialogue should be fun to read. It should flow easily as a conversation and sound like people are talking right in front of you. But too often it reads stilted and unnatural, like it’s been written down, not spoken. Some people have a natural ear for it, but most don’t.
The most common mistake with dialogue is to be too direct. People say what they mean and give full and frank explanations of themselves. It’s like a bank robbery where everything goes smoothly and everyone gets rich, there’s no fun in that. Dialogue is very flexible and you should take time to bend it. Lying, avoiding the truth (not the same thing), answering a question with a question, changing the subject, these are all ways of making a conversation interesting.
But my number one tip for improving your dialogue is this:
Setting the pace
A man is told on the phone that his girlfriend is in danger from the criminal types he used to hang out with. He rushes out of the house and notices how beautiful the flowers in the garden are.
This is going to slow the pace, but NOT because it is slowing down his journey to the car, which will get him to his girl etc.
Consider: The same man rushes out of his house to rescue his girlfriend, but he is intercepted by his parole officer who is there to check he isn’t consorting with nefarious types, otherwise it’s back to the slammer … What does he do now? (Can parole officers do that? I don’t know, I’m making this up).
Even though I am slowing down the character’s progress, I am not slowing the narrative. Because pace isn’t about how long it takes to get to the next thing, it’s how long it takes to get to the next interesting thing.