Don’t try too hard.
As ornate and complex as your exposition might be, don’t over think the dialogue. Sure, you might have certain characters that speak in riddles, or use scientific terms frequently, whatever it is, it’s a quirk, not the norm. People generally speak very simply, very straight forward. So if all of your characters speak without contractions or only use words longer than eight letters, you might want to rethink things.
Be real but don’t be boring.
Hellos, goodbyes, every day redundancies are implied. Don’t waste your time–or your reader’s–going through all the hellos, goodbyes, and I love yous. This applies to ums, uhs, likes, etc. No one wants to read that. These parts of speech have their place, absolutely, but don’t crowd your dialogue with them because you think it will sound more natural or authentic.
Punctuation is your friend.
It seems like no big deal, but punctuation can change the way a line reads. Its adds and takes away emphasis. First, you should know how to use your punctuation correctly, but once you have a good grasp, play around with it. Here’s an example: “Stop…” versus “Stop!” Moving commas and periods around changes the way a line reads just as much as ending punctuation does. Here’s another example: “Get. Away. From. Me.” versus “Get away from me!” The use of periods between each word implies a very intentional force. The reader isn’t thinking these observations consciously in their mind, but they will read it differently if you change the punctuation.
Go observe a conversation and take note of how many times someone interrupts someone else. It might seem rude, but that’s just how conversation works, it means all the parties involved are engaged in what’s happening. Characters can interrupt for an array of reasons including: disagreement with the speaker, a sense of urgency, feeling they already know what the character is going to say, excitement. The list goes on. Interruptions also keep your dialogue quick, and the reader will wonder how 50 pages flew by. Interruptions can additionally be useful for a change of subject, scene transition, and giving adequate “page time” to each of your characters. Interruptions can also be used to illustrate power dynamics, gender dynamics, age dynamics, you name it. The more characters you have, the more interruptions there should be.
There is such a thing as saying too much.
This is important both to the exposition and to action. It’s tempting to have your characters tell the audience the whole backstory. But it’s too convenient. Give your readers some credit, if you do it because it’s easy, they will know. Along the same lines, they don’t want to know everything all at once. Mystery is great. Your character might allude to events from their past but not outright state what they’re referring to: “Remember what happened to Dan last month?” “Oh, yes. Poor thing.” Your readers will keep reading to find out what happened to Dan, so down the line when you reveal what it is, they will feel satisfied, and hopefully, come to understand your story better in some way. You may also have your characters refer to a person or a place frequently, like, “The Lake” and every character knows what they’re talking about, but the reader doesn’t. Use that to your advantage. The easy guess is that “The Lake” is a lake, but what if it’s an underground fight club? That draws the reader in. As for action, it’s pretty simple. A character doesn’t need to say, “I’m going to cut the ham and make a sandwich” and then they cut the ham and make a sandwich. Actions speak louder than words.
Sometimes not saying something says it more clearly.
By this, I mean use subtext! Oftentimes people won’t say what they really mean, but they will imply it. A character might want to say “I love you,” but a lot of people are allergic to “I love you”. Instead, they might say, “I made you dinner.” It shows they care. Or maybe, “That song came on the radio today and it made me think of you.” Or, “Don’t leave, please.” The dialogue underneath is way more interesting than what is on the surface. In real life people hardly ever say what they mean, so why should your characters, right? When does, “Fine” ever mean fine? Not saying something can also apply to silence. Sometimes saying the wrong thing is great, but sometimes saying nothing is better.
Say it aloud.
It may sound dumb but do it. Just read your dialogue aloud to yourself (it’s actually beneficial to read all your writing aloud). The best way to tell if something sounds unnatural is to see how it feels in your mouth. Even better, have a friend read it aloud to you (or a tutor in the writing center!). Then you get to hear it and see if there are places where your reader stumbles or where something just isn’t working.