In the end, any story that will showcase your value to admissions will inevitably be able to be backed into one of the Common App essay prompts; because the whole point of these prompts is to help you identify insightful, revealing stories about yourself. In fact, a good personal essay topic often fits within the confines of multiple Common App essay prompt options. You might even identify a new and unexpected spin on your subject while considering it through the lens of a prompt you never expected to use (and may not have even considered in the first place). Over the years, so many surprises have revealed themselves through the Backwards Brainstorm, perhaps most of all that it works every time.
Good questions. Supplementals are aimed at making sure you really like their school and are familiar with it. Start with the big things that you like about the school, rather than things like “I live just a mile away.” What is the FEEL of the school? What do they value? What makes them your favorite? Prove to them that you are itching to go there. In order to do this, you will need to do your research on what is different about that school. They don’t expect you to know everything about it, but you darn well better know some things that separate them from other schools.