I read or watch or otherwise consume a thing and notice some character quirk that I think could be fun. I poke at an app. If I manage to do a complete app while still excited about the character -- and then don't get sidetracked or bored or beat to the punch by the time it's time to app them, I go for it!

This has resulted in no small number of short term characters, but hey, everyone's fun to play for at least a little while. . . .