The inner workings of one, how one gets indoctrinated, why they get indoctrinated.... etc... I'm a huge fan of watching documentaries Past Cultists escaping their cults (especially children cause they're the most abused form of a cult member.) and explaining their life as one.
Though, if you're going for a darker route, a bad end, so to speak, I am also a fan of those kinds of stories where the story at the end has them join the cult or become tainted and broken.