So you need a Problem to stick your character with, and you need realistic ways for the character to respond to the Problem in order to flesh out who the character is. But how the heck do you come up with the idea for the character in the first place?
I myself have two ways of going about it. Other people have different ways of creating character concepts, that work best for them. As usual with Advice On Writing, your mileage may vary.
Route 1: I’m reading, or watching, some other piece of media and one of the characters strikes a chord with me. So I pick up the character, examine what makes him appealing to me, I dust off a few of the character traits and add some from other sources, and voilà. Frankencharacter. I’ve also heard this called “filing off the serial numbers”, though I’m pretty sure that filing off the serial numbers applies to taking one specific character instead of creating an amalgamation.
So for example — and this is a character that’s rumbling around in my head, though you won’t see him for another few years, I think — take the emotional rigidity and stubbornness and sarcasm of Javert, and the weird mix of brashness and gentleness of Oblek from the Oracle Betrayed trilogy, and throw in a Tragic Past, and there you go! Put him in a new environment, and dress him up in different clothes, and if you know those characters already, I hope you’ll be able to recognize the influences — but he’ll still be his own entity, separate from the original bits and bobs.
I should note, at this point, that I might have an idea of what the amalgam character looks like, but probably no set image.
Route 2: comes at this from the opposite direction. There’s an actor just doing their own thing, and I think to myself, “self, I want to have this actor play a character based on one of my books.” It’s pure self indulgence, but it’s fun, and that’s why it works for me.
And hey, if Cornelia Funke did it with her Inkheart books, then so can I.
Sometimes I can’t figure out the character beyond the basic archetype, and that’s when I go to my friend and toss over a picture of the artist, and we have a brainstorming session — what sort of villain is he? What world would he fit in? And usually while we’re puzzling out the answers to those questions, Inspiration bops me on the head, and then we’re off.
Now we go find a Problem to stick that character in …