As you may have gathered, these are not to be viewed as hard-and-fast rules and there are occasions when some or all of this advice can be ignored. It very much depends upon your characters and your story. However, the following should be borne in mind.
1. Characters’ Speech Should Be Natural
In order to maintain the believability of your fictional world, it is advisable to model your character’s dialogue on normal, natural speech. In order to achieve this, it’s a great idea to spend some time listening to people talk; on the bus, in a restaurant or café, simply listen to conversation and get a feel for various styles and rhythms of speech.
That said, there are some facets of natural speech that should be avoided, as you’ll soon see in the points below.
2. Does Your Character’s Dialogue Have a Purpose?
In natural conversation, people often chime in with essentially irrelevant comments. However, in fiction, if your character is speaking just for the sake of speaking, it may be worth removing the passage. Generally, dialogue should always have a purpose.
So if the speech says nothing about the character or does little or nothing to advance the story‘s plot, you may want to consider removing it. Of course, in some cases, the fact that a character jibbers on meaninglessly may be the trait you wish to present (think Mrs Bennett in Pride and Prejudice), which is fine, but beware that this may make the character annoying or dislikeable.
3. Steer Clear of Clichés
Typically, authors are always on the look out for clichés and avoid them at all costs. However, it can be easy for a dialogue cliché to slip beneath that well tuned radar. Essentially, if it’s cheesy and seems contrived, your reader will instantly be able to see you at work - the thing we’re trying desperately to avoid.
Unless it is obvious that the cheese-factor is part of the character, in which case that needs to be clear in other facets of his or her representation, any kind of cliché is best avoided.
4. Avoid Excessive Hesitation
Again, depending upon your character, you may want to throw this ‘rule’ straight out of the window. However, consider the way you and the people around you speak. Chances are, there are a lot of pauses, ‘umms’ and ‘errs’ - if you transcribed a regular chat, you would notice the jerky, clumsy speech and immediately notice how ugly it seems on the printed page.
Therefore, you should avoid too much of these habits. That's not to say 'umms' and 'errs' shouldn't be used, but I would recommend using them sparingly. If you do choose to use them frequently, for one particular character, to illustrate his or her hesitancy as a personality trait, then it’s a good idea to avoid it in the story's other characters.
5. Get to The Point
Wherever possible, it is a good idea to avoid long, rambling passages of dialogue. Unless, of course, this is character-based. Usually though, in a basic fictional conversation, you’ll find that when editing your work, several words, or perhaps even whole sentences can be trimmed without losing the meaning. So, unless you have a specific reason for setting your character off on a longwinded monologue, it is a good idea to keep things as short and sweet as possible.
6. Make Sure The Words Suit The Character
While listening to others' conversations, you’ll have noticed the individualism of each person's speech habits. Consequently, you need to ensure that each of your characters has his or her own unique voice. It’s essential that they don’t all ‘sound’ the same, because, for a reader, that can become tideous very quickly.
Therefore, it can be very helpful to think about your character; where he or she is from, their culture, what accent they may have, possible speech impediments etc., etc. Make sure that any and all of these individual traits are reflected in the character’s dialogue. For example, if your protagonist is a working-class, London girl, who drops her aitches, make sure you write with this in mind. “Oi, I ’ad that!”