Let's start with some quick and easy tips, then move from there.
First: Follow colloquialisms. People in our time tend to speak with contractions. ("Why aren't you here? Don't do that. I can't.")
Don't info-dump. Writers sometimes use characters to relay information to the reader, but this doesn't work if both characters are repeating info they ALREADY knew.
Don't over-use OR under-use tags and beats. Not sure how to use them? Check out "Using tags and beats," an earlier chapter from this book.
Have you ever texted someone and realized the tone came off totally wrong? This happens in dialogue too! Notice the difference between this:
Do whatever you want." She smiled and handed him a cup of tea.
"Do whatever you want." He took a step away from her.
Conversations between people rarely have monologues, so there's no need for them in your book either. Although you don't need to (and shouldn't) include every nod and "Mhmm," remember that even stories get interrupted with questions and reactions. At the very least, the person talking isn't just sitting in one place. What are they doing? How do they look?
A question I see often is how much dialogue is appropriate. The answer: as much as needed to move the plot forward!
Dialogue breaks up long pages of chunky paragraphs, and it gives characters a chance to interact. More importantly, well-written dialogue tells you not only the information actually being conveyed, but also how these characters' relationship dynamics are.
Not everyone talks the same way, so make sure to distinguish the way each character speaks too. Some characters may have speech mannerisms like repeating "Honestly," before every sentence, or they might speak more self-deprecatingly than others do. The best conversations are ones that don't even need attributions (but have them anyway) because the ways characters speak are so distinct.
If you're writing a book in the real world or in a certain time period, make sure to write phrases (and slang or jargon) they actually used during that era. If this calls for research, don't shy away from it. Your WIP is worth that extra work.
Don't write out stutters, and for the most part, PLEASE don't write out "accents." It's distracting and often disrespectful, and it's much easier to just tell the reader someone speaks with a twang or dialect than to write it out.
Small stutters may be fine, but something like this is unnecessary: "I-I-I d-d-didn't m-m-mean t-t-to d-do th-that!"
(For more information, you'll be able to read more from my upcoming book I CAN'T BELIEVE I WROTE THAT: A SELF-EDITING GUIDE, which you can now add on Goodreads.)
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From an Editor: Common Writing MistakesNon-Fiction
As a content and copy editor, I see mistakes that many writers make. I've compiled these into some quick tips and examples for anyone who may find this helpful. (Adapted from a Twitter thread I wrote.)