Some years ago, I worked in the fraud department of a credit card company. Much of the work we did, outside of sending and requesting vast stacks of paper, involved comparing signatures and trying to figure out if people were lying to us. When things got slow (rarely) or management was otherwise occupied (almost always), I would pass the time by learning as much as I could about forensic handwriting analysis and document analysis without moving from my desk.
I came across descriptions of a fascinating technique called “statement analysis”. I would say it’s about as scientifically rigorous as graphology—i.e. not very—but the premise that how someone says something or what words they use can tell you what they think about what they’re saying is worth considering.
Creative writing, unlike criminal investigation, does not (generally) demand scientific rigour, so I think we can afford to look at one part of the theory which specifically applies to writing believable dialogue. This comes up not because I’ve been subjected as an audience member to unbelievable dialogue, but I’ve noticed one or two instances in plays where this principle was conspicuous in its absence. Since they were plays that paid special attention to the use of words, I paid special attention to the words used.
The theory is that a person will almost always use the same word to refer to the same thing (or person) unless they have a reason to do otherwise. This is called a personal dictionary or personal lexicon.
We see novice fiction writers violating this all over the place, possibly because some well-meaning English teacher (very likely not a published author) instilled in them the mania for using every word in their vocabulary as often as possible.
Here’s an example:
“You’ve met Sheila, haven’t you? My wife is a great admirer of yours. I was telling the other half just as we were leaving that we might run into you here. Dearest, this is John Bigslow.”
Reading it off the page (or the screen), you may find yourself wondering if “Sheila”, “my wife”, “the other half”, and “Dearest” are four different people. God help the actor stuck with that line in anything but a farce.
This is perhaps more natural:
“You’ve met my wife, haven’t you? She’s a great admirer of yours. I was telling her just as we were leaving that we might run into you here. Sheila, this is John Bigslow.” And then have our nameless doting husband refer to his wife as “my wife” in the third person and “Sheila” throughout the rest of the text—unless something catastrophic happens to their relationship, in which case the change of attitude can be subtly underscored by a change in language: “the wife” versus “my wife” works wonders.
Of course, this is an exaggerated example. However, on topics people feel strongly about, particularly those of politics, religion, and identity, they are utterly rigid about their vocabulary. What would you think of a person or character who used “Israeli” and “Jew” interchangeably? Do they think all Israelis are Jews and vice-versa? At least unconsciously. Do they think the distinction is important? Clearly not. Is this what you are trying to convey with the character? I hope so, otherwise you need to go back and clean up that dialogue before someone else reads it.
You can’t have someone explain the distinction between two terms, such as “sister” and “nun”, and then use them both to refer to the same person. Someone who self-identifies as “queer” or “African-American” is highly unlikely to switch suddenly to “gay” or “Black”. Even “kid” and “child” are decidedly different words; almost never will you hear the same person use both, and when they do, they mean two different things.
An exception to this principle is heightened or poetic dialogue, which is why Shakespeare is credited with adding so many words to the English language. Meter and rhyme call for a different approach to word choice. Just be sure you intend to be a poet before you commit poetry.
People actually use a smaller set of words than the average writer or playwright knows. This is to their advantage, as they can easier separate characters by varying their vocabulary space accordingly.
Never mind principles and rules. The best way to learn how to write effective, realistic dialogue is, of course, to listen to how people speak (this is why the best writers tend to be the least talkative people). Then you can evolve your own personal principles and rules of dialogue.