Does anyone know the best way to prepare for the interviews in DC? I have no idea what to expect
Anything on your transcript is fair game, with Newtonian Physics and Calculus the common thread for all majors.
If you're an engineering major, chances are, you'll have someone in your discipline interviewing you, and they're likely to ask you questions in your field. If you're something like, say, a Biology major, they'll resort to asking you questions like "Derive the formula for the area of a circle" or "Does my ass look fat in these pants?"