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. Her remarks also show 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 that has absolutely no relevance to the story. Everything said must be relevant, revealing the 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.
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 sometimes used to drag the story along with exposotion.
If the dialogue does not develop the plot or show the characters' personality or ambitions, then leave it out. Sometimes silence works best.
Conversations must hide instead of explain. Hide when you can reveal. Lie when you can tell the truth.
Allow your characters to misunderstand, to talk at cross purposes, to interrupt, to hesitate.
Direct the dialogue off stage. Call for the waiter to pay a bill, call to the bartender to order another beer. Allow the action to interrupt the flow.
Have your characters answer questions with questions. Like “Did you steal the picture?” with “What do you take me for?”
Allow your characters to talk to themselves. “What am I doing here?”
Just a few ideas to help you make your dialogue endlessly fascinating.
So you have to write a bit of dialogue. Where to start?
With the first draft, dialogue doesn't need to be subtle. Write on the nose. They say what they think regardless of consequences. Enjoy! This part can be fun. Just sit back and watch as the fireworks explode.
So what to do on the second draft? Two things. Watch what they do when they talk. The conversation isn't just about what they say; it is also about that they do. So pay attention. What do they do? Turn their back, sit down, squint?
Next, work out what they are hiding. Characters will always hide their true meaning behind their words. The subtext is important.
Learn from the masters. Watch a politician evading questions. Politicians can never be seen to be losing. Someone else is always to blame. They believe themselves to be the masters of deception. As if!
What the character says reveals more than pages of description. She reveals herself through her words; her thoughts, feelings and influence.
When she says “Politicians! They should be cut into pieces and fed to the lions” you know her better than if you had just written ‘She had extreme opinions on politicians.’