1) I'm in a Linguistics program if that helps...
2) I had an interview with my potential supervisor over Skype as part of the admissions process and she already had (and read) my research proposal, CV, etc but she wanted me to explain in my own words
A) my research project in detail
how I came about having this proposal
C) what implications do I predict my research would have
D) whether this research is just a curiosity or I am really interested in academia
E) what do I foresee myself doing with the PhD
Basically these are the things that you would answer in your SOP. I waited for the questions and answered them (upon hindsight, it felt more like an interrogation than an interview ). I thought I screwed up the interview really badly so I decided to wing it with the "Do you have any questions for me?" question by asking her what she thought about my research proposal, how I could improve it and (I still can't believe that I asked...) what are my chances of getting admitted. I thought I had nothing to lose anyway. She told me how to improve my research proposal and gave me the standard "our admissions is very competitive" spiel.
Well, I guess you could start by really reading up on the area that you are interested in working on, think about your research proposal more (particularly how you are going to conduct the research, the ethics involved, the implications, etc) and the implications of your research (how would your research contribute to the current academic community of your field). You could ask, very politely, on the professor's thoughts on your research.
I hope this helps and all the best in your interview!