And I'm going to have to break out the metaphors. Sorry about that.
If you think of storytelling as sewing, let's say, oh, quilt-making, theme is the embroidery. It's maybe not strictly necessary to the quilt. It doesn't make it warmer, or more durable, or make it more efficient. What it does do, however, is strengthen the seams, tie the disparate pieces together, and add texture and interest.
So you have this quilt, this narrative. How do you identify and then reinforce a theme? Well, metaphorically speaking, you start looking for motifs. Repeated patterns, repeated colors, repeated shapes. Echoes and parallels, especially in twos and threes. And then when you've identified your themes, you look for ways to reinforce them. You outline with a contrasting color to draw attention to them, or you overlay in satin stitch with the same color to puff them up a little and bring them to the viewer's attention. You look at the embroidery you've already done, and maybe if you've tied together two differently colored blocks with a chain stitch in a contrasting color, you pick another complementary color and use that to elaborate the chain stitch.
The thing is, you never want to overdo it. You can certainly state the theme, but it's much more effective to reinforce it subtly. And if you only reinforce it in one way, you run the risk of becoming didactic, and that is boring. Boring is bad. (Okay, some people read because they want their prejudices confirmed. There are plenty of books in the world for those people. Me, I want to read books that say, well, here's one angle, and here's another, and here's a third, and here's a solution, but here are the flaws to that solution, and but maybe--")
*g* I'll out myself a little here. I tend to use film and television as my examples for these discussions rather than fiction for two reasons. One is because a two hour film is a hell of a lot simpler than a four hundred page novel, narratively speaking. The other is because audience share is a lot higher for a film or a TV show than a book. (Somewhere back in history I have my Unforgiven vs. Casablanca discussion. I think it's in memories... oh, yes, here it is.)*
So this was where I was going when I was talking about Criminal Minds. Its basic thematic discussion relates to whether free will exists, and what personal responsibility is, and where it ends. And so we get these lovely complicated situations, consistently, where Our Heroes are put in a position where they have to choose. Do you save the life of somebody who you are confident will become a serial killer, although he has not yet? What do you do when you let somebody get away, either through weakness or just plain having made a mistake? What do you do about somebody whose obsessive-compulsive disorder or schizophrenia or reaction to childhood abuse causes them to commit crimes? What about somebody who was under the influence of hallucinogens administered by a poisoner? Or a cult leader? Where does the responsibility lie?
And by coming back to it again and again, from different angles, and by making each of the protagonists somebody who fits the profile of a serial offender, and by showing how the same violence affects the victims (the most gutwrenching scenes in the show tend to be when somebody who had a chance to intervene and didn't know it at the time finds out, later. I suspect the words most often uttered on the show are "what if?" and "if only..."). The protagonists, in other words, become the contrasting and complementary colors. What they're doing is elaborating on that theme, picking it up in different colors, stitching the edges of the blocks together and linking them and showing the thing in layers and lights.
And you can do the same thing when you write.
*Coolest thing about Unforgiven? Everybody in it is the same character shown in a different light.