So first, you should read your dialogue out loud and make sure it
doesn’t sound too formal, too casual, or in any way unsuitable for the
occasion.
William Sloane in The Craft of Writing offers some excellent advice:
There is a tentative rule that pertains to all fiction dialogue. It
must do more than one thing at a time or it is too inert for the
purposes of fiction. This may sound harsh, but I consider it an
essential discipline.
There shouldn’t be dialogue just for the sake of hearing your
characters talk. Every word they say should be an important discovery in
some way, even if it is just a casual conversation. I don’t mean that
every time a character speaks, they better be saying something crucially
important. You are allowed to write a conversations between fictional
characters that is fairly uneventful. But even an exchange that may seem
insignificant, if done right, will help build the reader’s image of the
character. Dialogue should either move the story forward or add to our
understanding.
Janet Burroway in Chapter 5 of Writing Fiction, A Guide to Narrative Craft points out something important about dialogue:
Dialogue can fall flat if characters define their feelings too
precisely and honestly, because the purpose of human exchange is to
conceal as well as to reveal - to impress, hurt, protect, seduce, or
reject.
Often, what is not being said is just as important as what is. Do not
try to overexplain things in your characters’ speech. Remember that a
lot can be expressed through their mannerisms and what they hide.
Please remember proper punctuation. This can kill your story. Just as
a quick overview: what a character says out loud should be within
quotation marks, and the period/exclamation point/question mark should
be INSIDE the ending quotation mark. If you are adding a tag, i.e. Mary said,
the comma goes before the quotation mark and the period comes after the
full sentence. Each dialogue spoken by a new speaker should begin in a
new paragraph.
Just as a final point, be careful with writing dialect. Burroway explains,
There is no point in spelling phonetically any word as it is
ordinarily pronounced… When you misspell these words in dialogue, you
indicate that the speaker is ignorant enough to spell these words in
dialogue. Even if you want to indicate ignorance, you may alienate the
reader by the means you choose to do so.
For example, in J.K. Rowling’s Harry Potter - though I love
love love the books - the way she wrote Hagrid’s speech was extremely
off-putting to me. Maybe it works for some people, but for the most
part, if it is something that can be conveyed in mannerisms don’t put it
in the dialogue.
As I said in the beginning, the best way to make sure your dialogue
is believable is to read it out loud. I hope this was helpful to any
budding writers out there!