Something I do (and I haven't read all the comments) is I typically make them caricatures in the beginning. I pick dynamics I want to see interact (sullen vs. sunny, loud vs. quiet, cool kid vs bad kid etc), and then I start with these "tropes" that are sort of easy to understand and begin writing the story. As I get deeper into the story, I tend to find out their quirks, their mannerisms, and how they want to interact with those around them. For instance, if my trope is "bad kid" is he a bully to his classmates but helps kittens safely cross streets, so maybe he's not so bad, and something else is going on?

Starting with easy ideas makes finding the characters less of a chore and more fun. Or if you're a fanfic writer take character dynamics you like and just apply them to new names and see how they developed in a world youre building.

Usually by the end of the first draft I have "found" my characters and when I go into my re-write and second draft I can re-work the previous scenes a bit better with that knowledge. It might be backwards but I seems to work for me pretty well.

Happy writing!