So you hear someone say that a book was full of expository writing, and you're not sure what they mean. Here's my simple explanation. I say simple because that's how I think. ;-)
There's a movie from the 80s called The Great Muppet Caper. Maybe you've seen it? Well, I'm old, I do admit it. There's a part in the movie when Miss Piggy is talking to her new boss, Lady Holiday. Lady Holiday is going on and on about her degenerate brother, Nicky, and Miss Piggy says, "Why are you telling me this?" Lady Holiday says, "It's plot exposition. It has to go somewhere."
What she's saying is this- the information about Nicky is stuff the viewer needs to know, and there's no other way for us to know it unless Lady Holiday talks about it. Would she, if it were real life, say that much to Miss Piggy about Nicky? Maybe or maybe not.
There have been books I've read in the past that have gone something like this:
(The two sisters, Mary and Ann, are talking about their mother's funeral.)
"As you know, Mary," Ann said, "mother hated roses. We cannot have roses at the funeral. When she was young, her brother, Humphrey, stomped on her rose bushes and told her that tending them was a worthless waste of her time."
I'm sure you can see the problem. (Besides the fact that Humphrey would have come out of those rose bushes severely wounded.) If Mary and Ann grew up in the same house together and had a traditional sister relationship, Mary already knows all that about their mother. Ann doesn't need to tell her. (The only reason I can think of that Ann would need to say that to her sister is if she's so scattered, nervous, sad, whatever, about their mother's death that she's just rambling to her sister. It would show that she's not thinking clearly or she's really upset.)
Sometimes in writing, it's tempting to put information that the reader needs to know into conversations between characters. Sometimes that's ok. Suppose Ann was telling a friend about the rose thing. That would make sense if the friend didn't know much about Ann's mom. Other times, though, it needs to go into the narrative. (The part of the story that's not dialogue.)
The reason some writers feel the need to put "plot exposition" into dialogue is because they're worried that there's too much narrative. We've all read paragraphs that have gone on forever, either with too much description when we want to get to the meat of the story, or too much backstory all at once. If a character is having a flashback, for instance, as a reader I don't want it to be a long one that will pull me out of the story's action.
One suggestion to avoid too much narrative is to intersperse it throughout the story. The reader doesn't need to know all at once every detail surrounding Mary and Ann's mother's dysfunctional relationship with Uncle Humphrey. In fact, if it's going to be an issue that comes into play later, it's information that needs to be spread throughout the story to keep the pace going and the suspense up.
So when you're writing dialogue, make sure you think carefully about the characters who are interacting. Ask yourself these questions:
* How well do the characters know each other?
* With the characteristics I've given them, how will they interact?
* Do I have them sharing useless information with each other, things they would already know?
Remember to avoid plot exposition in dialogue at all costs! Leave that to Lady Holiday and Miss Piggy. :-)