The depth of your emotional involvement in any storyline is directly related to the depth of the main character's back story. Want a strong screenplay? Develop your back stories!
When was the last time you watched a movie and found yourself immediately captivated by the storyline? Were you still captivated thirty minutes later? How about one hour later? If you cared about the main characters plight, then you were most likely watching a movie that had detailed, pertinent back stories for each of the main characters.
The successful blockbusters of any genre will have very detailed back stories for their main characters. Occasionally, a story with a set of shallow characters will rise to the top of the box office charts, but it's few and far between when that happens. Fleshed-out characters with storyline-related back stories are necessary for a strong film.
How many people do you talk to on a daily basis? How many other human beings do you interact with? Think of the one person you interact with on a daily basis but don't actually converse with a whole lot. How about the mailman? Or the clerk at the grocery store? Or maybe the java slinger at the local coffee shop? That one person with whom you have limited interaction with has an exhaustive, detailed back story. No doubt you know some of it, even with the minimal interaction you have with that person. The point is, no matter who a person is, he or she has a very complex, detailed back story to their life!
To properly convey a real person's back story on the screen would be virtually impossible. You'd need hours and hours of footage, and even then you'd only be scraping the surface of their life story. Now that you understand how in-depth a real person's life history is, take a close look at your characters and answer yourself an honest question: how deeply developed are your characters? If you look at them and give an honest perception of their history, you'll probably find out they're pretty shallow.
It's quite alright if your characters are shallow, at least during your editing process. You'll need to develop them further before you ever attempt to market the screenplay. And that doesn’t mean you'll need to cram useless details of their lift history into the storyline. What it does mean is that you'll need to include pertinent, storyline-related information about their past. The more you can tie their past in with the current storyline, the more enjoyable your story will become. The more related their life history is to the current plotline, the more believable the entire story will become to your audience, and the more they will care about the outcome.
If you were writing about a modern day burglar, his back story should reveal (as well as reinforce) his current means of earning a living: burglarizing homes. You could reveal how his father taught him how to steal from the local grocery store together as a team because the family had no money when he was growing up. You could reveal all sorts of sordid details about his past that could add to the credibility of the story. The more the character's history coincides with his past, the more the audience will begin to care. It's that simple.
One other bit of advice: not only should his past relate to the storyline, but his past decisions should also relate to the storyline - or better yet: his past decisions are the cause of his dilemma, and he must not only solve his current problems, but he must also learn to change his nature at the same time!
Ultimately, the development of your characters coincides with the development of your story. They go hand in hand. The more time you spend developing your storyline, the more developed your characters will become. It's unavoidable. Good luck with your writing!
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