You have two or more characters in a room and they need to talk. How do you make it believable and compelling and not stunted and boring?
It can be tricky. First of all you have to remember that dialogue is there for a reason. It's meant to move the story forward. It's not a place filler.
Here's a list of dos and don'ts to help you along.
Do:
1) use dialogue to reveal character. Instead of saying your character is angry or selfish, show it by what they say and how they say it.
2)use dialogue to catch the reader up on back story. Sometimes it's more interesting that way.
3) use dialogue to create tension. It should never be smooth sailing between your characters. There should always be some underlining tension or foreshadowing of tension in what is said and how it's said.
4) use only the words you really need (see point 1 in the Don't list). Keep dialogue precise and to the point.
Don't:
1) try to write dialogue the way we actually speak. In real life we use a lot of filler sounds, like "um", "like" and half sentences. It's okay to use certain slang words sparingly to emphasis a character trait, but the key word here is sparingly.
2) let all your characters speak in the same manner. Especially watch out that they don't all speak like you. :)
3) forget to add action in your scenes of dialogue. People are usually doing something while conversing, even if it's just making an expression with their faces.
4) use fancy dialogue tags like "exclaimed" and "shouted." This should be obvious in they way they say something. "Said" is the best tag because the eye just skims over it and it doesn't jolt the reader. Also acceptable in moderation are tags like "asked," "replied," "prompted," "stated," and "probed."
Great dialogue reveals great character. How each of your characters speak reveal who they are and hopefully they’re interesting! And unique to the other characters around them.
Here's a scene from Leif Enger's terrific novel, PEACE LIKE A RIVER. His two main characters, a brother and sister called Ruben and Swede are butchering a goose. Watch how he reveals character and builds tension through their dialogue. Notice that they aren't just talking at each other, but DOING something at the same time. Both the talking and the doing reveal character and create tension.
Rueben's POV:
I had, of all things, a lump in my throat. Luckily Swede was standing at my elbow and said, "First thing, you have to cut his head off."
"Well, I know that."
She prodded the goose with her finger; plucked , it looked pimply and regretful.
"Then the wings," she said.
"You want to clean him?"
Swede let it go and stepped over to the ruins of a grain truck that had been parked behind the barn to rot. She shinnied up the big rumplike fender and sat there with the wind tugging her hair. It was a cutting wind; the light was leaking from a mottled yellow sky. Imagine a sick child all jaundiced and dirty about the cheeks--that's how the sky looked. I picked up Davy's knife and tried it against my thumb, then beheaded the snow.
Watching, Swede said, "Forgive me running, Rube?"
"What?"
"I ran away."
"From the goose? Swede, it wasn't any big deal." I tossed the head into a cardboard box we'd found in the barn and went to work on the wings. They came off a lot harder than the head; I had to saw the knife blade back and forth.
"Come on, forgive me," she insisted.
I nodded, but said nothing. Those wings were gristly fellows.
"Out loud," she said.
She was the most resolute penitent I ever saw. "Swede, I forgive you. Is it all right now?"
She hugged her elbow. "Thanks, Reuben--can I have the feet?"
I whacked them off at a chop apiece and tossed them up to the truck. Swede caught them and scrambled over to the grainbed. My hands were freezing and I dreaded the next part--I ought to've taken Davy's offer to clean the goose. Aiming at a spot under the breastbone, I plunged in.
"Swede," I said--just talking so she'd stay with me--"I don't get what's wrong with Davy."
She didn't answer right away. She sat on the flatbed toying with the goose feet. She took so long to speak I got involved in a tangle of guts and forgot I'd said anything.
Finally she said, "He's mad about Dolly."
"Oh." Davy's girlfriend. "How come?"
She looked at me. "You heard," she said. "Last night, driving over."
We'd gotten a late start, as I mentioned. The football team had been busy getting whomped; it was almost eleven before we got on the road.
"I was sleeping."
"You were faking, I could tell. Just like me."
Here's a short scene from LIKE CLOCKWORK (companion book to Clockwise)
Adeline goes on a drive in 1955 with a guy she's just met.
We pulled into a lookout with a view of the Hollywood valley. A couple of kids in the car next to us were making out. I hoped Howard didn’t have any ideas. I wasn’t ready to let our relationship go that far yet.
Especially since we weren’t in a relationship.
Howard motioned for me to follow him out of the truck. We leaned up against the front bumper watching as the streetlights in the valley below started to pop on.
Howard shoved his fists in his pockets. “How old are you, anyway?”
“Eighteen.” It just slipped out. I couldn’t believe I'd just lied about my age.
“Eighteen,” Howard said, like he was rolling the number around in his mouth. “Are you sure about that?”
It was my ponytail. It made me look too young. “I think I know my own age,” I insisted.
“Well, then, would you like a beer?” Howard stepped over to the side of his truck and reached in to open a cooler.
“Eighteen’s not drinking age,” I said. Besides I hated the taste of beer. I shook my head.
“Who’s gonna tell?” Howard had a can opener on his key chain and used it to remove the cap off the bottle. He took a swig then eyed me with a tilt of his head. “I don’t really like girls who drink beer, anyway. Not very attractive.”
So in summary: dialogue, used together with action, reveals character and backstory, and creates tension compelling the story forward.