Imbalance can take many forms in a plot. It can be an imbalance of power, and power can take many forms: physical power, intellectual power, emotional power, just for starters. It might be an imbalance of status: academic, cultural, societal. It might be an imbalance of emotions, where one character loves/hates a second character, while this second character doesn’t feel that emotion at all. All these sorts of imbalance can be used to create tension, tension creates conflict, and conflict is what pushes a plot into motion. Imbalance can create interesting contrasts in your characters and settings.
Need an example? Think of Charles Dickens’ Oliver Twist, with the plot revolving around social inequality like a wheel around an axel. Dickens was to revisit this theme again and again. All of Oliver’s adventures are due his poverty, and he would have had no escape if it wasn’t for the fact he was the scion of a comfortable family. The ethical and moral issues created by poverty were underlined by the fact that Dickens’ protagonist had suffered needlessly. Life for the poor in Victorian London was monumentally unfair, but poor Oliver didn’t experience more bad luck than most. Dickens didn’t need to highlight an unbalance that existed in real life. And he did give Oliver a happy-ever-after.
It is easy to create an imbalance, but it is harder to create an effective imbalance. Dickens knew what is was about, and you should take note of his technique. He made the imbalance clear from the start. You want to provide tension straight off.
Terry Pratchett jumps head first into creating imbalances in his books. In I shall Wear Midnight, the very first scene sets up Tiffany as a powerful witch, but not one respected in her own community because of her lack of age. As well, the boy who had been her ‘romantic interest’ in the previous Tiffany books is getting married to someone else, and the community knows it. He is minor nobility marrying another minor noble; she is a farm girl. Imbalance on top of imbalance on top of imbalance, a tower of conflict that exploded into a plot of epic proportions. (I picked this book at random from my shelves … feel free to make your own observations.)
In another example from Terry Pratchett, look at how he has used imbalances to create the character of Sam Vimes. Same breathes and grows because of these imbalances in his life. He is a poor man who has married a very wealthy woman. He is a policeman who is now a political force in his city and in his culture; and like all policemen, he dislikes the way politics interferes with justice and the law. He is a man who thinks with his feet – and if you can’t see the beauty in that trait, you need to look harder. The Sam Vimes in Guards, Guards is a very different man to the one in Raising Steam; however, his basic personality hasn’t changed, he has just grown with time and experience.
You can use even imbalances to create a contrast in a setting. The messy bedroom versus the tidy office. The City looming over the Rural landscape. The sewer running under a mansion…
The Industrial Revolution was all about imbalances created by technology. As a writer in the Steampunk genre, the contrast between Science and superstition fascinates me. As well, I concentrate on the imbalance of rights and power between Victorian-era men and women. I’m certain you can think of a dozen examples on your own.