Many writers think of movies as visual books: not only are features about 120 pages long, many novels have in fact been made into good movies (The Accidental Tourist, Postcards from the Edge.) But the truth is, movies are more like haikus than epic sagas: one strong idea carries the experience.
So how do you know if your idea will make a good movie? Here are three areas to evaluate:
This should be your first test: is this a story you have to write? It's hard for an unproduced writer to sell an idea without a killer script to show it off, so you'll be investing six months to a year in developing, writing, and rewriting your script. If you are not passionate about telling the tale, you won't have the necessary energy to see you through the inevitable obstacles. At the very least, is this a story you'd be willing to get up an hour early every day to work on? Also keep in mind that some day you'll be selling your idea to production companies and that's a little like leading the charge into battle. If it's not running through your blood that this, above all others, is a story that must be told, you won't be able to get other people fired up about the project.
I know, this sounds like some kind of creepy Valentine's spider, but bear with me. By "legs" I mean, do you have enough plot to carry 110 pages -- and minutes? This is clear-cut: no matter what you write, you need to have a beginning, a middle and an end; there need to be surprises, twists, payoffs, victories and defeat. Stuff needs to happen.
How much stuff is enough? To some extent, it depends on the genre (Raiders of the Lost Ark vs. While You Were Sleeping), but you do need at least seven major moments for your main character. For instance, take While You Were Sleeping: In the opening scenes, Lucy is established as a character, then 1) she saves Peter's life; 2) she's mistaken for his fiancee; 3) she meets his brother Jack, who is determined to "out" her; 4) people mistakenly think she's pregnant; 5) Peter wakes up; 6) she agrees to marry him, even though she loves Jack; and 7) the right man proposes to her and we get a happy ending.
The first incident sets the story in motion, the second complicates it and sets it off in a new direction, the third complicates it further, the fourth complicates it at a point where the third complication is winding down, the fifth launches the third act, the sixth is the moment where all is lost, and the seventh resolves the story.
Other things are going on, of course -- in fact, throughout most of the story, Lucy and Jack are falling in love. But that's not a specific plot twist, that's the emotional core of the movie. Which brings us to "heart."
You need to know what your script is about, emotionally, at its core. We don't go to the theater to be informed, we go to have an emotional experience. If you have any doubt about that, think of football games: no one goes to a game just to keep track of the score. You go because of the varied emotional experiences: rooting for your team to win, suffering when the quarterback throws an interception, rising up in righteous anger at a bad call, being part of a group united in a common goal. Football, roller coasters, movies... all are forms of entertainment and engagement on a visceral level. Even if you've written a teen slasher flick, your idea must have an underlying theme that touches our humanity. That's why Frankenstein's monster still resonates more than a hundred years and as many incarnations later: it may be a horror story, but its core is the universal desire and fear of man's power over life and death. It connects with our hearts and our heads.
Finally, if you want to make money writing movies, you need to write movies that will sell. Let me be clear: I believe anything can sell if exceptionally well-executed (hey, something as off-beat as Adaptation merited an Oscar nomination). But it's foolish not to look at the 'business' side of show business. What genre is the story? Thrillers are easier to sell than dramas, which are often based on previously-published books. What's the budget? For writers just breaking in, low-budget scripts have more doors open to them than something with a dozen exotic locales or major explosions. Is there a strong male lead? Sexist it may be, but actors get more projects made than actresses. Is there a romantic element? Comedy? Sex still sells, and laugh-out-loud funny trumps almost anything.
So what do you do when your idea fails to live up to one of the above? If you are lacking passion, look for another idea immediately; if you don't love it, it's not going to turn out well, no matter how marketable it may seem. If it's plot or heart you lack, that can be brainstormed, pounded into existence, uncovered or created. Read books, talk to fellow writers, and if you're still stuck, crib from the great, universal stories, from Cinderella to Shakespeare.
As for whether or not it'll sell, try to stack the odds in your favor. However, if you're passionate about the story, and the idea has both a strong plot and an underlying, human element, then I'd say go ahead and write it even if it's not an easy sale. You're a writer because there are stories you are burning to tell. Never let anyone -- least of all me -- talk you out of following your passion wherever it may lead.
Hmmm, that sounds like a good idea for a movie...