WARNING: The following post contains plot spoilers for the movie Thor: The Dark World.
Dark characters in a work of fiction are those who err more on the criminal end of the spectrum than the law-abiding end. Think villains, bullies, antagonists, bad boys, reluctant heroes, rebels, and even sometimes foils, sidekicks, and minions. These character types (roles) can get flat, static, and predictable pretty quickly.
Here’s the fix.
Anyone who saw the first Thor movie will think of Loki, the Norse god of mischief and Thor's adopted brother, as a villain.
That's probably fair. He spent that whole movie deceiving his family and conspiring against several Marvel kingdoms, including his childhood home, Asgard.
With that background in mind, viewers of Thor: The Dark World, the second movie about these characters, will automatically assume the worst of Loki.
This time around, though, one event—one catalyst—gives Loki an extra dimension beyond that innate ability to make trouble.
His mother Frigga is murdered.
Until that turning point, Loki's practically a stereotypical villain, and almost as two-dimensional. (Picture a cartoon “bad guy” rubbing his hands together and cackling manically.) He plots against everybody who might get in the way of HIS plans and goals. No one else figures into the equation.
Which makes his transformation—from selfish, cocksure criminal to genuinely devastated and grieving son—all the more powerful.
Heck, as he slumps against the far wall of his prison cell, with the furniture smashed around him, his own bloody footprints laying tracks across the floor, his hair a mess, and a hopeless lethargy weighing him down, he looks ... human.
And the audience starts to care, which complicates everything even more. He's become a three-dimensional villain, and someone with whom we can sympathize.
Isn't he really more dangerous, more unpredictable, that way?
What's the "chink" in your dark character's armor? What does he regret? What does she fear losing? Where could you add even one or two scenes that show your dark character's other side?
A more complex dark character means more involvement from your readers.