Think about the people you know. When you start a conversation with one of your friends, you have certain expectations. You’re familiar with the kinds of things they say in various situations, and how they say them. You tend to have a good idea of how they converse. You can tell when they’re in a weird mood because they tend to say things that are out of character.
Characters have voices. You can open a well-written book and read a conversation, and deduce things about the personalities behind the words. There’s an art to establishing character voices, of course. I know I could do better myself, as sometimes I catch myself just having all my characters talk like I do. There are some things that I try to keep in mind while writing dialogue that may help you.
Keep in mind that different people have different vocabularies. Your education, line of work, areas of interest, social circles, and age are some of the factors that influence what words you use in casual conversation. You personally might be studying anatomy and physiology intensively, but the character you’re writing about doesn’t necessarily know the technical names for his different bones or muscles. If you are writing about an expert in a certain field that you aren’t so familiar with, study a bit and pick up some useful terms. Think about you’re character’s social life, and the time period when they probably established most of their conversational skills. Watch out that you don’t have an old person who uses a lot of millennial slang, unless he’s trying to be that way, or something.
Which brings us to the subject of period writing. I tend to be critical of historical fiction. I wouldn’t exactly recommend imitating the way people talk in historical fiction paperbacks. I’d recommend reading some original sources from the time period in question. Don’t assume that the more awkward and wordy a sentence is, the more authentically historical it will be. You might be surprised by the informality that sometimes shows up in dialogue from the past. Take note of the words used, and be careful not to fall into using obviously modern expressions.
The setting where your story unfolds also will have profound effect on dialogue. Phrases and terms are different in different countries, and even different regions of countries, sometimes. We all know a few U.K. terms: the trunk of a car is a boot and the hood is a bonnet, etc. Americans might make mountains out of mole hills, but a Pole can make an ox out of nothing, top that.
It isn’t important to obsess over regional things like that all the time, casual readers probably won’t notice or care. What any reader will have opinions about is the use of dialect cues and phonetic spellings when it comes to foreign accents. My feeling here: just don’t do it. Go ahead and tell your reader that a character has a certain accent when they talk. Don’t try to write it down. Why? Because it almost never captures the essence of the accent portrayed. For example, I once read a book by one Jean Stratton-Porter called Freckles. (The only romance I’ve ever choked down.) In it there was a Scotch-Irish couple. Apparently, the word “well” with that accent sounds like “Wheel.” Somebody help me here. It that right?
Even if the author is pretty good at it, (like Mark Twain) it tends to be really hard to read when all the words are grossly misspelled. It can be so distracting for an important character to have an accent that it detracts from the whole book. (At least for me.) Listening to a foreign accent is one thing, trying to decipher it in print is quite another.
I just wrote a book (A Hand with Five Fingers) in which the main character was from the United States, but everyone else was foreign. There was an Englishman, a German, and three Hungarians besides. Imagine the fun I could have had with phonetic spellings and how chaotic that might have turned out.
Lastly, I want to point out how much the character’s intrinsic personality makes a difference in their speech. I wrote a series that spans three books so far called the Rhapsody Threnody Series tentatively, and in my opinion the characters in it are some of the strongest I’ve ever developed.
The main character, Carmen Hess is cynical, and often depressed, but is carried through by her resolve and sense of humor. The books are written in first-person from her perspective, and therefore the whole narrative as well as her own dialogue are in her voice. She utilizes deadpan sarcasm and a lot of negative asides, and the whole text is embellished with weird analogies and unusual observations.
Her best friend Candace Leonardi is colorful, upbeat, and always ruining Carmen’s bad moods. She tends to tease other characters good-naturedly, and uses a lot of informal language. She calls people things like “tiger” and “chiquita” and tells them not to give her “that jive”.
Cecil Vorosvari, Carmen’s young ambitious piano teacher is emotional, obsessive, sensitive, and highly intelligent. He’s also foreign, and throughout the series is speaking his second language. His English is very good however, and he only “losses his English” on occasion, and, true to his personality, he is extremely eloquent. Another note about foreigners: just because they might not know a lot of English, doesn’t mean they don’t use complex words. Depending on the language, complex words may actually be easier to understand for them because they’re often similar to a related word in their own language.
Now, this has been an incredibly long post. Sorry. I hope it was helpful enough to make up for it. As one final tip I advise that you read your dialogue aloud to yourself without the narrative to see how it sounds. Your ears don’t lie. If it sounds wrong, it is wrong. Tweak it, you’ll get it.