Poor dialogue happens when the writer isn’t sensitive to the way his character talks. Poor dialogue is when characters overact and the dialogue is explicit. Poor dialogue is when it is used to drag the story along.
If the dialogue does not develop the plot or show the characters’ personality or ambitions, then leave it out. Sometimes silence works best.
What your character thinks is shown in everything she says. She may be proving or disproving some particular point or enunciating some universal proposition. All these comments reveal her personality. This also reveals her moral purpose. There are things she wants to talk about or things she wishes to avoid.
Don’t confuse the reader by having your character talk about something of absolutely no relevance to the story. Everything said must be relevant, revealing character’s motivation, experience and attitude.
Why have characters waste interaction just to cover, say, travel arrangements? In general, use dialogue to heighten suspense and tension, not to tell us what we already know.
Think about releasing the dialogue by allowing your character to clarify; backtracking a little over what they have just said. Think also about repetition, interruption, changing the subject and echoing.
Then listen to how dialogue is refracted by the physical world. Cars honking. Animals braying. Plates breaking. All these sounds get in the way and give the dialogue an extra depth.
Avoid people talking in a vacuum! It must be perfectly clear where your characters are and what they are doing while they are talking.
Give the reader a chance to breathe and keep his interest alive by removing the obvious answers.
Create thrills between the lines.
Just as in real life conversations, allow your characters to misunderstand, talk across purposes, change topics of conversation, get angry, interrupt each other, hesitate, answer questions with questions.
Remember that many conversations are meant to hide instead of highlight and explain the reality. Thus good dialogue hides in silence and lies instead of revealing the truth.
Show us, don’t tell us, what’s going on and why. Don’t write ‘She was loud and rude’ but “Get outa my way, you jerk!’ she bellowed.
Is the character interested by what’s going on? Keep her active – no sitting around, doing nothing as she might walk off stage. Where does she go? Round out the character by showing complex psychology and emotions such as fear, frustration, anger, rage and envy.
Dialogue is like going out on a date. You never say exactly what you mean but you hint at what you want. You dance around the question instead of coming right out and asking.
Listen to each character speak. If you can’t hear your characters speak, then most probably what you want them to say, most often ought not be said.
What differentiates each character is usually their word choice. This is key in shaping their voices. Forget the sound of the voice, forget the accent, forget pace. Think word choice.