In order to create a three-dimensional character, we need to individuate, or integrate, their archetypal parts into a cohesive sort of unit-y whole. This, of course, includes both their persona and their shadow.
Now, the mark of a good villain in any story falls under his ability to force his opposition, the hero, into the spotlight where he'll find ways to highlight and criticise the things your hero would like to hide.
Now, Shadow: Recognising It
The shadow is upsetting the acknowledge, so we shove our awareness of it down to an unconscious level, thereby making the only way to truly know the contents of your shadow to consider everything that infuriates you, disgusts you, horrifies you beyond compare. For example, Jung would say that if cruelty made your character sick to their core, that cruelty was their shadow.
I know what you're thinking: does that make them a cruel person?
No, of course not. But it does mean that they will have a bitch of a time in accepting that they're capable of the kind of cruelty that sickens them. Your characters repel their shadows, just as you'd repel yours.
So, Find Your Hero's Shadow (And, In Turn, Your Villain's)
Try not to over think it, because your unconscious does better work.
I'd start by listing the qualities and values that makes your hero a hero. So, is he brave or selfless or stand up for the voiceless? You can get some words for characteristics on this, Sandy Tritt's Personality Components chart.
Then, I'd list the qualities and values that makes your villain a villain. Forget all this shadow business, and just come up with around five - like they're vengeful, or dishonest or power hungry.
And then, next to each quality or value you've written for your hero, write the exact opposite. Alright, so let's say that your hero has charm, so then your villain could possess opposing traits such as rudeness, pushiness, abrasiveness, clumsiness or crudity.
Add a behaviour then, to the right of your list of opposites, that demonstrates your non-heroic value. For instance, to show their rudeness, they could tell crude jokes or make disparaging remarks toward people.
So, compare your hero's shadow to your villain's characteristics. So, that list you made of what makes your villain a villain, see if any of them match the opposite traits of your hero. Since the opposite qualities are your hero's shadow, they should be personified by your villain.
Villains Personifying Heroes' Shadows
A good villain is the dark side of your hero, and the greatest danger your hero faces should be that under the right pressures and given certain circumstances, they could embrace the very qualities that make a villain a villain. (Really, at some point, your hero should start to do exactly that too, even if by accident).
If your villain's qualities are the things your hero hates - even if it scares them - then they'll do anything to bring the villain down, even if it means becoming the villain. But we must remember that shadow qualities are what infuriate you, what makes you sick. We are all drawn to oppose, to fight, the things which we hate, which means that your villain can become a nemesis, but really only if the villain themselves, down to their character and behaviour, arouse an obsessive drive in your hero.
Take Batman and the Joker, for example. There is always that fear, that denial, that Batman could down spiral into the maniac insanity that the Joker embodies, and so he fights it continually, to the point where it becomes clear that if Batman kills the Joker, he will have flipped.
But How Thin Is the Line Between?
We all know about that thin line between hero and villain, and in the end, it's all up to one choice: the hero chooses not to become his shadow, and instead acknowledges and integrates his shadow qualities into the rest of his personality. Sometimes, the villain is a fallen hero, someone who would've been just like the hero had they resisted the draw of evil.
Okay, so some examples.
In THE LORD OF THE RINGS, the Ring draws each character's shadow to the forefront. In the Matrix, Neo has to become Agent Smith and embrace his own shadow to overcome to machine world. In Se7en Mills-also-known-as-Brad-Pitt not only becomes wrath but in doing so, he falls prey to his own shadow by becoming the killer he's pursued.
Finally, How Heroes Fall When Villains Push Them Over
Say you want to move your hero from good into the grey area, and the trick to doing so is in the villain pushing his proverbial buttons. So, let's return to Batman and the Joker. Justice without killing is the most important thing to Batman, and the Joker continually prods him, trying him to break his only rule.
The obvious reaction is rage, and not just at the Joker, but at himself, with the frustrations of not being able to overcome the Joker without succumbing to his shadow (because when the Joker gets locked up, it's a holiday). The more justice Batman isn't able to accomplish, the more hurt and anger involved, and the more drawn he is to the final solution. If he does kill the Joker, a part of the foundation on which he bases his identity is destroyed, and unless he would be able to acknowledge those uglier parts of himself, he'd risk becoming the villain.
So what do you think, comme? Do you know what your main character's shadow is? What do you think about the interaction of the hero and villain over this thin line?