Whenever I see a liquid I know is poison, some deviant bit of my brain will whisper, "Drink it."
It's scary. I don't want to die or be sick, and yet I have to consciously curtail this abrupt impulse to drink that seductively sparkling liquid. I've never studied psychology, but I believe this creepy, whispering bit of my brain is named the id.
What makes it relevant to writing (besides being something good and weird to draw on) is an idea I came across in someone else's blog, which I have unfortunately lost the link to now. Whoever you are, I'm so sorry--you're totally brilliant.
The idea they put forward is that everyone has an id, or a "shadow", including your protagonist. Their shadow contains all the bits of themself that they hate and fear most--everything they don't want to face.
Usually, a person's shadow is invisible to them; they don't know the shape of their own darkness. So why is the idea of a person's shadow relevant to how you portray your protagonist? Because there is one particular reaction that does show off a person's shadow:
When we see our id embodied in another person, we are so terrified of it we lash out at that person with an irrational anger. In other words, we most furiously hate the things we are most in danger of becoming.
If your antagonist embodies your protagonist's id, then your protagonist will be almost obsessively motivated to stop them. And what makes that conflict really juicy is your protagonist's worst impulses are those of their shadow, so as your protagonist becomes more stressed, they will have to fight all the harder to keep themselves from becoming just like their enemy.
Their internal struggle will mirror the external one, and they'll be forced to examine those parts of their own character that frighten them most. It's a great way to force a personal growth arc onto your protagonist.
Does your protagonist have a dark side? Is it mirror of their antagonist? Would you or have you ever set up that symmetry on purpose in a story?
Have you seen the shape of your own darkness? Does your brain ever give you disturbing suggestions your conscious mind rebels against? Can you see parallels between the things that makes you angriest and the things you would be capable of doing if you gave in to your worst impulses?
I'd love to hear your thoughts.