Bad or unrealistic dialogue is something that can cause a book with a good plot to crumble. Stunted conversations aren’t interesting to read, but in the same way, flawless conversations with characters who always know what to say aren’t realistic either.
Each character should have his/her own way of speaking. Realistic dialogue makes for more realistic characters. Listen to the way people you know speak. The words people use can tell you a lot about someone. Think about how your characters are going to talk when you’re creating them. What is common of their culture? What have they grown up hearing? Have they been to school or not? How old are they?
Often I will see characters using big words. While these words can look impressive, they’re not always necessary. In fact, coming across a big word when reading can cause you to stop, halting the flow of the writing.
Attitudes come in to play here. Does your character hesitate before they speak? Do they stutter or mispronounce words? Does your character swear? How do they react when people around them swear? A simple conversation can reveal a lot of things about your characters.
Try not putting who says what. How the statement is being made should show which character is saying it. The first time you hear a character talk, you can describe what their voice sounds like. Practice. You don’t have to include all of these conversations in your actual book. Have your characters discuss controversial topics. Some people don’t talk a lot, and that can show things about them as well.
Writing accents. This is something sometimes I’ll struggle with when writing. I can hear the [insert nationality here] accent in my head, but it doesn’t translate to the page. Some authors will misspell words to show accents. Sometimes this works, but often times if I see a book like this, I’ll put it back.
I’ve found the best way to write accents is by looking carefully at what they say, not how they say it. For instance, if I’m reading a book and one of the characters refers to a TV as “telly” it’s an automatic note to me that they’re probably British (I’m sure this works the same way to someone who is not American, vice versa). Someone who speaks English but lives near a different country, might have some of that language integrated into their language. You can look up what people from different places say, but often you will still end up messing up a few words and sayings. There is an infinite amount of “British slang” lists on the web.