Desire, John Truby, The Anatomy of Story, p. 43–45

Once the weakness and need have been decided you must give the hero desire. Desire is what your hero wants in the story, his particular goal.
A story doesn’t become interesting to the audience until the desire comes into play. Think of the desire as the story track that the audience “rides along”. Everyone gets on the “train” with the hero, and they all go after the goal together. Desire is the driving force in the story, the line from which everything else hangs.
Desire is intimately connected to need. In most stories, when the hero accomplishes his goal, he also fulfills his need. Let’s look at a simples example from nature. A lion is hungry and needs food (a physical need). He sees a herd of antelope go by and spots a young one that he wants (desire). If he can catch the little antelope, he won’t be hungry anymore. End of story.
One of the biggest mistakes a writer can make is to confuse need and desire or to think of them as a single step. They are in fact two unique story steps that form the beginning of your story, so you have to be clear about the function of each.
Need has to do with overcoming a weakness within the character. A hero with a need is always paralyzed in some way at the beginning of the story by his weakness. Desire is a goal outside the character. Once the hero comes up with his desire, he is moving in a particular direction and taking actions to reach his goal.
Need and desire also have different functions in relation to the audience. Need lets the audience see how the hero must change to have a better life. It is the key to the whole story, but it remains hidden, under the surface. Desire gives the audience something to want along with the hero, something they can all be moving toward through the various twists and turns — even digressions — of the story. Desire is on the surface and is what the audience thinks the story is about.
Let’s look at some story examples to see the crucial difference between need and desire.
Need: Hero John Miller must do his duty in spite of his fear (psychological and moral)
Desire: He wants to find Private Ryan and bring him back alive.
Need: Each of the men in the group needs to regain his self-respect (psychological).
Desire: They want to make a lot of money by performing naked in front of a roomful of women.
Need: The hero must regain his self-respect (psychological) and learn to act with justice toward others (moral).
Desire: As in all courtroom dramas, he wants to win the case.
Need: Jake must overcome his cocky arrogance and learn to trust others (psychological). He also has to stop using people for money and bring a murderer to justice because it is the right thing to do (moral).
Desire: As in all detective stories, Jake’s desire is to solve a mystery — in this case, to find out who killed Hollis and why.
KEY POINT: Your hero’s true desire is what he wants in this story, not what he wants in life.
For example, the hero in Saving Private Ryan wants to stop fighting, go home and be with his family. But that isn’t what tracks this particular story. His goal in this story, requiring him to take a series of very specific actions, is to bring back Private Ryan.
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