Make your characters three-dimensional and up the stakes in your story by giving each character a dark secret, something to hide.
To be honest, it's no fun to read about characters who have absolutely nothing to hide, no sordid pasts, no closeted skeletons, no illegitimate children or abusive upbringings or buried treasures.
When you give your characters each a secret to protect, a secret that they really don't want to have to share, you accomplish two things: you round out the characters, and you up the stakes in your story.
Round Out a Character
By the halfway point of Tangled, you think you know Flynn Rider pretty well: bad boy, reluctant hero, typical greedy thief and ruffian.
Yeah, that about sums him up.
But a lot of those characteristics, paired together, are pretty predictable. What if you wanted to give Flynn an extra dimension, something unexpected that readers won't see coming?
You give Flynn a secret, something he really doesn't want anyone else to find out, and then you put him in an impossible situation he doesn't expect to survive, so he feels compelled to share that one personal secret about himself. (You'll have to watch the movie to find out what it is.)
And now you've seen a deeper side of Flynn than you expected. He's become more endearing, less standoffish, more human ... and more well-rounded.
Up the Stakes
Rapunzel's secret is her power to magically keep people young (even to heal wounds) if they sing a certain song and are touching her enchanted hair. For a while, that's almost the only thing you know about Rapunzel, but the secret deepens the story because her hair becomes the thing that keeps her in bondage to her mother (and prison warden).
When Rapunzel runs away with Flynn to pursue her own dream, her mother does everything she can to get Rapunzel (and her hair) back safely. And Rapunzel's hair (which magically glows when the certain song is sung) helps get her and Flynn out of some tight spots.
As long as Rapunzel carries this secret with her, as long as she can keep people young and beautiful, she'll never be free from those whose greatest desire (at any cost) is youthfulness. The stakes are high for Rapunzel, and the odds don't look good for her escaping her fate. Viewers will hang on till the bitter end to see what happens.
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Don't hesitate to give your characters a secret, something they wouldn't want anyone else to find out. Maybe she's a convicted felon, or he grew up an orphan, or she's got cancer, or he's working out a hidden agenda.
A secret is a gem whose mining will deepen your story in many different ways.