Well to be honest, I draw a lot. (I actually got a small gig with Funimation XD) but when a character I sketch out has a cool vibe to them I start expanding, what're their likes? Dislikes? Faults? Peevs? And triggers?
Mostly though I wing it. I had an idea from where I wanted to start from (revenge) and I decided to try and think of forshadowing, what they've been through and what they have to cope with... they're personalities. The faults and fears all things I'm afraid of or insecurities my friends have ranted to me about or things I've felt before. Their strengths are all things I see in the people I'm around or qualities I wish I had. The things I want to be and the person I am. Sometimes they're 2 different people but on paper (or wattpad) I can combine them. "It All Started With Revenge" was the first story I was really able to stary getting into but I slowed down because I lost my motivation... ironically enough this rant with you kinda makes me wanna get back to it. :)