Will check out that video!

Another one I remember enjoying (it's been a few years!) is the three-pronged theory of character development talked about in an older episode of Writing Excuses (https://writingexcuses.com/writing-excuses-9-13-three-prong-character-developmen...), which theorizes that sympathy, competence, and proactivity make characters compelling, and that treating them as "slider bars" and considering the effects of various configurations on those sliders can be useful when you're beginning to craft a character.