I've seen this play out so many times, and honestly, it's often where the most memorable fics come from. When writers take a tiny, unexplored corner of a character's backstory or personality and just... build a whole internal logic around it, the relationships stop feeling like plot devices. They start feeling inevitable.
Take something simple, like a character being afraid of the dark because of a childhood incident the canon never mentioned. That's not just a random quirk. It means they might need to leave a light on, which could annoy a pragmatic roommate character. Or it could mean they get jumpy on night watches, causing a more protective companion to adjust their own behavior without even commenting on it. The relationship isn't just 'they are friends'; it becomes a dance of unspoken accommodations and understood weaknesses.
Some of my favorite fics are built on headcanons about how characters communicate. Maybe one of them is secretly terrible with words, so they show affection through actions—mending clothes, leaving out a favorite snack. The other character might misinterpret this as indifference for chapters until the moment it clicks. That slow realization, built on a headcanon about their love language, creates way more tension and payoff than just having them confess feelings outright.
It works for antagonistic relationships too. A headcanon about why a villain is so cruel—maybe stemming from a specific humiliation or loss—doesn't excuse their actions, but it gives the hero a more complex person to oppose. Their conflict becomes a clash of philosophies rooted in personal history, not just good guy vs. bad guy. I've read enemies-to-lovers stories that only worked because the author planted a headcanon seed early on about what the 'enemy' truly valued, making the eventual shift feel earned, not forced.
In the end, it's about giving the characters private rooms in their own minds that only the other character, and the reader, get to slowly discover.