They just appear in my head and I usually have their physical description locked down in a heartbeat. Like in this one MG backburner I have, the MC is a dark-skinned 10-11-ish black girl with her hair cut in a sassy little 20's bob updated with sideswept bangs, wearing a blue and white tankini, playing at the beach, wears a seashell bracelet on her ankle as an anklet because she's so small she can still get away with it. She has a plain face (not remarkably pretty but not ugly either) but the sweet, kind and hopeful expression on her face tends to warm people up to her more than they would to a scowling grumpy beauty. She is gentle and averse to confrontation. A bit fearful of getting picked on by the bullies.
See, at that point I'm done with what my mind's eye saw and now I need to paint a picture of who she is mentally and emotionally and that usually involves puling more characters out of my butt to provide context.
-Are her parents happily together or did her mom die in a freak car accident years ago? -Are her parents intellectual college professors or do they own a used car dealership where they spend every moment hustling to get ever last penny out of their customers pockets using their slick snake oil salesman charms? (Which is a different kind of intelligence to being a professor but just as powerful if not more so!) -Are there siblings? If so, what are the dynamics there? -We mentioned school bullies, what's up with that?
For me, all the extra characters that might not even make it to the book end up creating the MC's character AND can end up writing a huge chunk of the plot for me.
Oh, by the way, it always weirds me out when characters randomly pop into my head because for example, I know full well I've never seen a little girl with that exact description on the beach. In fact, the blue and white tankini is inspired by the hooker dress in Pretty Woman! The anklet story was me at that age. I've never seen any little girl of any race with the hairdo I described in real life. So how did my brain create this character without me even being involved?
Do you guys get that weird feeling too? Any theories?