Okay, as a title, it's a little cryptic, but here's what I mean: No matter what you're writing, even if you consider yourself well-versed in the subject, you will need to do research, and you will find out all kinds of fascinating things. You should include none of these in the actual novel unless it is necessary, essential, and absolutely required to the plot. Period.
Why? Anything else will make the novel feel like it's there to teach, rather than tell story and will instantly cause the reader to put down the book or, in extreme cases, throw it at the wall. (But, Greg, you say, isn't the point of children's literature to teach? No.). Also, in many cases, information that is new to you will already be so well-known to the character that they won't even comment on it -- tossing in the added stuff will not ring true to the character.
Here's an example: Suppose I were writing a novel set in Austin, over Labor Day weekend. I have my characters, native Austinites (call them Artemis and Athena) go into a mall and they happen upon an Abercrombie and Fitch store (For those who don't know what Abercrombie and Fitch is, it is a store from somewhere in the northeast which makes a catalog showing WASP-y college-age persons not wearing the clothes the store sells.).
Now, Artemis and Athena likely will be wearing whatever their fashion preference is that allows them to be comfortable when the temperature outside is in the mid-nineties. This is because summer in Austin lasts until about mid-November. However, going into A&F store, they will notice that the clothes on display do not include anything they could possibly wear for warm weather, and instead, include a lot of wool and thick, long-sleeved cotton sweatshirt-type garments. (Actually, this starts happening in late July).
Apparently, the reason for this odd phenomenon is that the fashion industry is based in the northeast, where they have these things called "seasons" and apparently do not believe in regional marketing (Hint to any clothiers reading this: you could make a fortune by shipping all your leftover summer stuff down south and not bringing out your fall line south of the Mason-Dixon until around December)
Now, I admit this is a lot of information. If I'm writing a scene, in which for some reason, the only thing that is important is that Artemis and Athena know that A&F sells winter clothing during September, how do I handle it?
There are several ways.
1. Artemis and Athena walked into Abercronbie and Fitch. Artemis was looking for a new pair of thong sandals. All she could see, though, was wool and winter boots lined with fur!
"Ugh," she said. "I don't believe it! Why can't I get summer clothes?"
"Oh," Athena replied. "It's because the fashion industry is based in New York. People up north are already wearing jeans and sweatshirts. They don't realize it can still be as hot here even in November as it is up there in July."
Here we've learned a great deal but at the expense of stilted dialogue (the "dialogue of great explanation" is almost always to be avoided), but we also have an unrealistic setup--Artemis, being a native Austinite, would already know about the wool and fashion phenomenon.
2. Artemis and Athena walked past Abercrombie and Fitch without pausing. They knew that after July, the only thing you could buy there was wool. As early as midsummer, the New York-based company had shipped all their back-to-school clothing to all their stores across the country. Because the company's home, northeastern market suffered dreadful winters, and even autumn was not necessarily free of frigid temperatures and freezing rain, they maintained (and brought out early) an extensive line of warm woolen and thick cotton clothing. Apparently, they didn't realize that temperatures in Austin during November could be as hot as those up north in July.
This one doesn't have that awkward dialogue problem, but is all that information really necessary?
3. Artemis and Athena walked past Abercrombie and Fitch without pausing. They knew that after July, the only thing you could buy there was wool.
Better, but is July significant?
4. Artemis and Athena walked past Abercrombie and Fitch, ignoring the wool.
Now, in this one, I've imparted several things: 1. The clothes at A&F came from a sheep; and 2, Artemis and Athena know enough about it to bypass the particular store.
The only thing I haven't done is explain everything to my reader. Should I? Only if I don't trust my reader.
To be continued. (Sorry, have to go to the airport and pick up my wife's cousin.)