I love a good series. I tend to write in series and I have ones that I keep on my bookshelf and revisit time and time again.
What is the pull of a series? The world. When the author has multiple books to work across, they have time to build a world of characters and culture that readers love to visit repeatedly.
Side characters that would normally be shallow and nearly meaningless gain more depth and appeal as they appear across multiple books.
In romances, readers get the opportunity to glance at their favorite couples' happily ever afters even as they watch the next couple fall in love. The deeper the world and tighter knit the characters, the more it feels like coming home.
So what is the key element to tie a series of books into the same world?
I watched my first Marvel superhero last year. The hubs and I were granted a rare weekend to ourselves. It didn't work out for us to go anywhere for the weekend, so we decided to just have movie night at home. We watched The Avengers and I was hooked. The weekend turned into a movie marathon and we watched Thor, Captain America, Iron Man, Iron Man 2, and The Incredible Hulk. Iron Man 3 was between theater and rentability at the time so we didn't watch it.
More recently we started watching Agents of SHIELD on television.
That was when I realized the true strength of the world Marvel was building. Samuel L. Jackson was on my TV screen and I wasn't watching a movie. Granted he was on there for only moments. It probably wasn't more than one day on set for him, but just that little touch tied the show so closely to the movies that you knew you were in the same world.
I would have thought that Phil Coulson, being a main character in both The Avengers and the television show would have been enough, but it was Nick Fury that really connected them for me.
For authors, I think that proves the importance of the side characters in building a world. It isn't enough to take a secondary character from one book and make them the main character in the next book. It's the little people moving in the background that really put your stories in the same world.
If you've followed me for a while you know I'm a big Julia Quinn fan. She has a remarkable example of a minor character making it evident that all your characters swirl around in the same fictional pot.
Lady Danbury appears as an eccentric, sentimental old lady in How To Marry a Marquis. She then pops up for a page or two throughout the Bridgerton series and the Smythe-Smith series. If you read all of Julia Quinn's books then you know and love Lady Danbury and seeing her again makes you smile. If you only read the one book, she provides a splash of comic relief in the guise of the wisecracking matriarch that you both fear and love.
Lady Danbury and Nick Fury are both stereotypical enough that you can feel like you know them in a just a few moments, yet developed enough over time that they feel real.
That is what you need in the background of your series. Secondary characters that the reader of a single book can appreciate but regular readers can love. Build a couple of those into your series and see a greater depth in your fictional world.