Let's talk about theme. We've already discussed building characters; now let's talk about the meaning of the characters' stories.
Sidenote: my upper and lower back are all kinds of messed up, and my feet and hands are semi-numb at the moment. So forgive me if I sound a jumbled mess.
So in any fiction novel, the plot is what happens - the physical bones of the story, the action - and the theme refers to the meaning of the plot. The theme is the representation of the human spirit within the book; it's what makes the story compelling for us.
Take superhero stories, for example (I'm looking at you, Marvel Studios, and y'all have no idea how much restraint it took just now to keep from mentioning particular films, and particular characters. Whoa. I almost fangirled, but I pulled it together). Yes, the special effects may be visually stunning, and yes, the action is exciting, but those aren't the reasons those kinds of stories are so wildly popular. It's because, amid the action, spectacle, and spandex, there are some deep, meaty, universal themes at play. Themes like personal sacrifice, the responsibility that comes with power, friendship, acceptance, relevance, self-loathing, and coming to terms with your own demons and shortcomings. Superhero stories are stories about personal growth; that's what we connect to as readers and viewers.
When we set out to write a story, we have to know who our characters are, and how they're going to grow. What themes will drive their growth?
I have a lot of fun, in my Dartmoor books, playing with the theme of acceptance. What does it mean to belong? How can you be accepted by the family while retaining your own identity? On its face, Fearless is about a rather taboo love story unfolding amid an outlaw biker soap opera. That's the plot of the book. But what it's truly about is the importance of family, the need for love and acceptance, and the persevering, rationalizing human ability to find joy amid the strangest of circumstances.
So don't forget the heart of your book. Even the most interesting of concepts needs a solid backing of theme to make the novel real and relatable.