To write and sell a movie it has to be a great story idea you can tantalizingly tell about in a sentence. Great is not something you luck into or because you are uniquely one of the outstanding geniuses. It is simple, and it is learnable.
To know what you are doing, if you want to write a great movie that embroils the audience, you must grasp the concept of the one-liner. Furthermore, to sell it to another, you must begin with a well defined story line that can be summed up in literally one or two sentences. That is the "one-liner" as it is called in the industry. This is so outrageously simple that when you have learned the knack of it you can rattle off great story lines at will.
The successful screenplay has much conflict to embroil the audience. This might be the oppositional gap between the good and the evil or the right and the wrong of the protagonist and antagonist. Simply a wide divergence in the main character himself or herself can cause conflict in its incongruity and unique paradox. The more diverse, the more unexpected, sardonic or contrary, all believably so, the more potentially explosive the plot. If you can't state the irony and theatrical appeal in a few sentences you need to rethink the story.
It isn't just opposites or contrariness of personnel. It is some paradoxical circumstance one wouldn't expect, haven't heard of before, somehow made credible by the characters and their personal drama. You'll see this in all the best movies.
I'll make up an example. A scandalous banker, now jobless in a crashed economy, gets a job peddling hot dogs from a push cart. That is sardonic. Even more ironical, it is in his own financial district where he is known by everybody, where nobody but him desires him to be back on top. Or this incongruity for a comedy: he is a white man, bank President, fired for mis-managed loans, and the sales territory he is assigned on his new job for a moving company is in foreclosure haven Harlem.
You should be able to reduce your spec screenplay to an antiphrasis stated in a line or several. A script that won't so reduce is probably flat or deficient in its plot or story lines and not a sellable work. Having a poignant one-liner the movie will practically write itself.
That is really the simplicity of the one-liner. When you have it it rings. It grabs your interest.
It says the story is unique and the description makes you want to see it. You can write a whole story from this cutting statement. Assuming one has craftsmanship, a studied skill, and imaginative rendition, which anyone can practice, one can develop a very good script package from the great one-liner . Even more, you can sell it in one sentence, which you must.
When the Producer who could make your movie asks what it is about you had very well better be able to tell him, now, with no fluster, and no ramble through scenes of the movie. This ultra condensed description actually demonstrates in a moment if you have a great story idea or not. One can see the strength of his own screenplay, but this is vitally necessary to pitch your movie to a potential buyer or producer.
Practice describing great movies you have seen in this way. Then make up some of your own.
Here is how I would one-line these movies:
A homeless, social misfit, super hero who has been thrown in jail by society for his destructive offenses must be asked by their Mayor to save the town from destruction. (Hancock)
A man zapped by a UFO turns on unusually perceptive extra-sensory powers and turns off all his friends who are freaked out by his abnormal feats, while he attracts unscrupulous government people who want to own his abilities. (Phenomena)
Twins from a biological experiment reunite but have turned out to be complete opposites physically and morally. (Twins)
A real Angel with wings gets his last sabbatical in an Earth body and is bent on having some rogue and rowdy Earth fun, but he's not here to touch any lives or perform any miracles. (Michael)
A man and his wife, unbeknown to the each other, are each secret agent assassins who become deadly enemies when they get opposing assignments. (Mr. & Mrs. Smith).
Wouldn't you want to see any of these movies only on the strength of the one-liner? Even without knowing the A-List actors who play the leads?
See these one-liners as brief, to-the-point statements of the real irony of the movie that make it a unique conceptual package.
Get it? Go to it.