What really helped me to articulate my interest for my supplemental application was researching areas of nursing that interested me. For me it was the opportunity to travel, to work in many different areas and specialties, if I choose to change midcareer I won't have to restart 4 yrs of university again, the opportunity for continuing education and advancement. I really focused in on the areas that interested me. Do you like kids, look into peds nursing, and read threads on this board about what nurses love/hate about that area, oncology, emergency, school, community/public health etc.
I'd say go into the interview with an idea on where you want to end up, but with the understanding that that may change in school. That way you'll come off at your interview like you know what you want, and why you want it. It might also help you tough it out in school, when the going gets rough.
Best of luck