basically, headcanon is your interpretation of a character :) in the fiction/drama world, it's the ideas of a character that you derive based off the source material, but those ideas may not necessary be explicitly stated. eg. i think that harry potter is the kind of guy who would be a bummer at college parties, but harry has never gone to college.

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for rl headcanons, the only thing we know about a star's character is what they choose to show us, so we use what they give us as building blocks and form our own opinions of what they are like. in my case, i think of jongin as a little gamer fluff ball who loves puppies. someone else might think of him as a cold, mean playboy. those are different headcanons.