People talk. The characters in your novel talk, but when you are using dialogue in a work of fiction, those spoken words have more uses than simple filler between blocks of narration.
In an article published in the Irish Times, three authors discuss the art of writing dialogue and how they use conversations between characters to develop the story and move the plot along.
As a writer, you can use dialogue to convey information to the reader, but as author George Saunders point out, the sentences have to be natural and conform to normal speech. If you learn nothing else, this one bit of advice could save you from a steady stream of rejection letters that begin as soon as the literary agent reads the first conversation.
Listen as people talk to each other, whether it's eavesdropping on strangers at the shopping mall or your family gathered around a holiday table. There is a rhythm to speech, a way of using words that can be simple or complex. Is your character talking to a child or an adult? The language would be different, and you have to write it in keeping with what you have heard. Two men chatting will not sound the same as two women arguing, and neither would the conversation be the same between mixed couples. You can convy a great deal about your characters through the words they speak, without requiring long stretches of description that bore the reader.
What is the key to writing believable dialogue?
You, as a writer, must listen.