To put money where my mouth is:
The Abyss, Your Skin, Your Soul
Each Skin has a different sort of relationship with the abyss. Some of this is hard-coded into the options for each Skin, but a lot of it is left open to interpretation. Ask about it. Ask, "So, how do your kind deal with abyss? Is it responsible for the way you are?" Ask leading questions, and let the answers imply a sprawling mythology about the world.
But don't get stuck there, on that surface level. Ask questions that personalize one's relationship with the abyss. Ask, "What about your relationship with the abyss sets you apart from other ghouls?" Ask, "Why does the abyss want to help you so much?" Make them feel special, because teenagers feeling like they're special in the eyes of some great unfeeling entity is fertile ground for drama.
If you do it right, you'll create two dysfunctional patterns. First, you'll put the Skins at odds with one another, based on differences in worldview and perspective. Second, you'll have lucy treating the abyss like a second boyfriend, and jack-o treating it like a limitless credit card, and you'll create messy dependencies. And the whole situation will just be a tower of mis-communications and presuppositions, waiting to collapse.