People have very keen ears for dialogue. They know what sounds right and what sounds strange without having to think about it. That only makes sense; all of us have spent countless thousands of hours both talking to and listening to other people.

You’ve probably seen that recent research finding 10,000 hours of practice at anything makes someone an expert. Do the math: You likely racked up 10,000 hours of listening, developing your ear for dialogue, by the time you were five years old.

No wonder so many writers say that writing good dialogue is one of the hardest parts of fiction: every single reader is an expert in the subject! If your protagonist is a boat builder or a bricklayer or a horse trainer, very few readers will have enough expertise in those fields to know whether you’ve portrayed those activities well. But the second your characters start talking to each other, every one of your readers will know immediately whether your dialogue works.

This is why dialogue is such a powerful tool for creating vivid, believable characters, and showing aspects of their personalities. It is also why a single bad line of dialogue can sabotage hundreds of pages worth of laborious character development.

Some examples are in order. Imagine that Steve and Dan have decided to take a year off between high school and college to go have adventures traveling in Europe. Near the end of their year abroad, they find themselves in Spain. Hard up for money and desperately seeking an alternative to what he perceives as a boring future back home, Steve enters an amateur bull-fighting competition. He does well, placing third and winning fifty Euros. However, the bull does well too, and Steve ends up with a few cracked ribs and a black eye. That evening, Steve spends his winnings on a hotel room and a few beers to share with his friend:

Steve tipped his head back, wincing, and drained the last of his cerveza. He took up a fresh one and held the cool bottle to his swollen eye. “What do you think, do I look like a bullfighter?”

“I think you look like shit, man.”

I intentionally told you nothing about Dan in setting the scene for that brief exchange. Dan gets exactly one short line of dialogue. What does it tell us about him? Quite a lot.

Casual swearing suggests that Dan is not a particularly formal guy. He chooses not to stoke Steve’s ego, but rather, to tell Steve the truth, suggesting that Dan is his own man with his own opinions, and isn’t afraid to share them.

I have not indicated how Dan delivers his line: you could read it as a humorous jibe, or as a frank and serious assessment. Both options imply something about Dan’s personality. Is he glibly laughing off the situation, or is he trying to help Steve see the seriousness of it? Either way, Dan is conveying a subtext of “Dude, don’t be an idiot. This is not a realistic career path for you.” It’s a short line, but it sure gives us an insight into Dan.

Let’s take a different example. Ellen and Julia have become fast friends after Ellen joined the coffee shop Julia works at. They are both in their mid-20s, post-college, figuring out how to deal with careers and boyfriends, how to make lives for themselves. One evening, Ellen calls Julia. After some opening chit-chat, Julia gets the feeling all is not well:

The line was quiet for a while. Julia asked “Hey, are you ok? You sound upset.”

Ellen sighed. “Rick stormed out. Hours ago. He’s not usually gone so long.”

“You guys fight too much. What happened this time?”

“I don’t know. I was cooking. I don’t even know what we were arguing about. I got really upset, and I threw a knife at him.”

“Ellen!”

“Relax, he ducked. I didn’t hit him. Anyway, he took off.”

“Well, I expect he’ll be back soon enough. Don’t sweat it.”

Let us further imagine that Julia is the protagonist of this novel. The author presumably intends for Julia to be a strong, sympathetic character—someone with a good head on her shoulders. That may be the image the author has worked to craft for Julia, but what does this exchange show about her?

Let’s actually start with what it shows about Ellen. It shows that Ellen probably doesn’t know when to get out of a bad relationship. It shows that she isn’t very good at controlling her reactions, and that she is prone to violent outbursts.

I don’t know about you, but Ellen is not the sort of person I would particularly want to have as a friend. Yet, supposedly she and Julia are good friends. Why? Why does Julia choose to be friends with a scary and dangerous person? It’s not like they have history. Ellen hasn’t given Julia a kidney. They haven’t known each other since kindergarten. There’s nothing like that, so Julia’s casual dismissal of Ellen’s actions at the end of the dialogue does enormous damage the reader’s view of her as a sensible person.

I’m not saying your characters should never say anything stupid. Sometimes they should, because they’re only human and people make mistakes. Sometimes that’s an important part of the story.

But I am saying that writers need to be very sensitive to the implications of the words they put into their characters mouths. If you make your characters say something dumb, it had better be on purpose because what they say says a lot about them.

The good news is that you are entirely well equipped to handle these nuances of dialogue. Remember, you’ve put in your tens of thousands of hours of practice at both speaking and listening, just like every one of your readers. You’re an expert, just like they are. Be confident in your expertise, and apply it to ensure that what your characters say conveys what you want it to.