Really, the prelim interviews for me have been just like the neuro ones. Very easy-going. They will ask why this prelim programs and why you think a prelim is important etc. All of the bread and butter questions. Not to mention, there will likely be other prelims interviewing with you as well. On my days, I've had both cats and prelims at one interview. Be sure you make an effort to talk to some prelim interns to get their perspective. Good luck!