I think it was the script guru Syd Field who observed that, in the old days it didn’t take much to satisfy film fans. A typical 1930s Tarzan movie plot might go along the lines of:
Tarzan is happy living in the jungle — An evil hunter enters the jungle — Tarzan defeats the evil hunter — Everything goes back to normal.
However, as audiences became more sophisticated, they became less satisfied with these simple linear plots and expected something more 'twisty' and interesting.
The problem of predictability has always been less true of books (where there is a long history of imaginative story structure) but you'll admit that even now there are plenty of lacklustre novels whose plots run on predictable tramlines that you can second-guess from the start.
To best way to avoid the mundane and predictable is to make sure your story is well peppered with Complications and Reversals.
Complications are the problems that your protagonist encounters when they try to solve whatever crisis you’ve burdened them with. In any good story the problems will pile up, whether it’s an action movie, play or romantic novel. The complications start off small and get larger and larger. Each problem raises the stakes — the penalty for failure becomes greater and greater.
Take the movie First Blood (based on a novel by David Morrell). At the start of the story, the ex-vet John Rambo is picked on by a small-town sheriff and his deputies. Rambo beats up the lawmen, escapes from jail and tries to run for the hills. This isn’t made easy, the writer throws in all sorts of complications to keep Rambo on his toes. First, Rambo has to clothe himself to stop him freezing to death; then he has to fight a posse; then eliminate a pack of dogs; then he has to dodge a helicopter; then he injures himself falling out of a tree...
More genteel complications can be found in Sense and Sensibility where the elder Dashwood sisters are searching for suitable husbands. However, just like Rambo, many problems are strewn in their path. First, their father dies leaving them without money for a dowry; then they’re thrown out of their house; then they have to move miles away to the wilds of Devon; then they discover their chosen partners are promised to other people...
Reversals and Changing Goals
Reversals are also problems, but on a bigger scale, to the extent that they often force a change in the protagonist’s gaol. Giving your main character a fresh objective is an excellent way of maintaining the surprise and excitement in a story.
In First Blood the appearance of the National Guard represents a reversal. Up till then, Rambo had been doing quite well for himself. He’d beaten everyone sent against him and had looked on top of the situation. So the writer calls in the National Guard to put Rambo in his place. As it turns out, the National Guard don’t defeat Rambo, but they do make life very difficult for him – they blow up a mountainside and trap him in an old mine. Rambo now has another complication to solve – how to escape from a labyrinth of claustrophobic, rat-infested tunnels.
After he escapes from the mine, Rambo’s reversal leads to a change in goal. He could easily slip away, but instead he sneaks back to the town and wreaks havoc. Rambo’s original goal was escape; now his goal is revenge. He wants to get his own back on the people who have hunted him so ruthlessly and the society he believes has rejected him. His thirst for revenge overrides his instinct for self preservation. Rambo’s escape at this point would have been an anti-climax, but his change of heart maintains the pace of the story and gives it an unexpected twist.
In Sense and Sensibility both sisters suffer reversals. The eldest Dashwood girl is devastated when she finds her gentleman friend is affianced to someone else, and her sister’s hopes are dashed when her young man moves to London. However, not all stories involve changing goals, if in Sense and Sensibility the Dashwood sisters suddenly gave up their pursuit of husbands to become clog dancers the reader would feel cheated (not to say confused).
More examples of changing goals:
In The Lion, the Witch and the Wardrobe, the children’s original goal (to get home) is overtaken by a greater one — to help Aslan fight the forces of darkness.
In Babe, the piglet’s original goal is to be accepted as a sheep-pig by the farm community. His goal changes when he’s entered into a sheep-dog trial; now he has to find a way to win and prevent the Farmer's humiliation.
In Ben Hur, the goal of our hero is to revenge himself on Messala, the Roman who sent him to the galleys. After Messala is defeated, Ben Hur is given another goal: to rescue his mother and sister from the Valley of the Lepers.
If you think your story is lacking fizz, have a look at the outline and see if you've made your protagonist's life too easy. What other banana skins could they reasonably be expected to slip on as they pursue their goal?
If you think your story is predictable, think about changing the protagonist's goal, what's the biggest plum they could aim to pick, and what motivates them to go for it?