Last night while flipping the challenges I stopped and listened to a bit about the lyrebird and was totally amazed to hear how accurately this winged creature could imitate sounds. Just like the mocking bird does other bird calls. This leads me to think about dialogue. When we write dialogue in a story, we're imitating the conversations we hear between people. Usually we clean them up and remove all the extraneous bits the ums, the ahas, and the wells to name a few. Dialogue in fiction is often not true to what we hear in everyday conversations. This is not to say it's artificial, for the most part. Some writers have a knack for making each of their characters speak with a different voice. Sometimes the speeches of the characters could be interchanged with the speakers and no one would notice. So just what use does dialogue have in fiction.
Dialogue should do one of three things or all three if you are a clever writer. Dialogue should define character, give needed information or progress the plot.
As you revise each scene, look at the dialogue to see if it's necessary. Throwaway dialogue has no purpose in the plot. Check to see if the person speaking has become an individual. Does his or her words show suck things as social standing, education, age, choice of career. Also look at your tag lines. Using adverbs can tell you what a character is thinking as he speaks but it's better to let your words show what the character is thinking or feeling.
When Jane and I wrote Becoming Your Own Critique Partner, we titled the chapter on dialogue as Listen to the Mocking Bird. That's what a writer should do when using dialogue.