By Janice Hardy, @Janice_Hardy
Maybe it's just that I'm working on the last book in a trilogy, but ways to handle back story have been popping up left and right. My newest revelation: when adding new details, don't reference stuff that happened in the other books unless it's important to the new plot, or a critical part of the world mechanics.
I've found myself mentioning things that happened in books one and two, not in a "this is what happened" way, but like private jokes between the characters. Comparisons or reminders of events past. They feel terribly natural because they do reference real things that happened, but for those who might be reading the series in non-chronological order, they won't have a clue what I'm talking about. Even those that are reading right along might not get it if it's been a while between books.
It's really easy to add these kinds of details, then on a later read, decide you need to add more exposition so readers will get it. Your instincts are right in saying "there's a problem here" but they can send you in the wrong direction. In most cases, instead of adding, it's better to subtract. Doubly so if these details are there to enhance the world building or character development. It might have had great significance in an earlier book, but without the context from that book to back it up, it doesn't always work the same way.
The spots where these little memories work well is when that memory triggers a natural way to have your protag talk about necessary back story. For example, in Blue Fire, Nya looks back and sees the Healers' League (I won't say more for those who haven't read it yet). She's instantly reminded of what happened there in book one, which gives me a great opportunity to introduce that part for new readers. And since what happened directly affects the problem she finds herself in at that moment, it advances the story as well.
But I also had a line that made a joke about something that happened in book one that had no bearing whatsoever on the plot. It was specific enough that it was clear there was more to it, but not clear enough to get it without reading the first book (provided they even remembered it). Readers would most likely feel like they were supposed to get it, but missed something. And you certainly don't want readers stopping to flip back to re-read, especially when the answer isn't there.
This is a tip mainly for sequels, but I'd imagine it has some usefulness in a first or stand alone book as well. If you find yourself feeling the need to explain an offhand comment or detail, odds are it doesn't need to be there. If that detail provides an opportunity to naturally talk about back story, it's probably fine. It can be tough to decide which is which sometimes, but if you look at what will advance the story and what won't, it should be a lot clearer.