There is a free pdf file entitled "Job Interview Preparation Guide", written specifically for EMS personnel, and listing most all of the common interview questions, at this site:
http://ems-safety.com/emsfiles.htm
Very highly recommended. I have linked that site here before, and will do so many times in the future because they simply put out so many excellent resources for EMS professionals. I also highly recommend the podcast audio sessions, where some of the best minds in EMS are interviewed, including Dr. Bryan Bledsoe, the author of the Brady paramedic texts. Check them out here:
http://ems-safety.com/audio.htm
Back to the question at hand, my favorite question of potential employees is simply why they chose to enter the EMS field. This is one of those questions where it is especially important to answer brutally honestly and completley, because I call BS on those who only say they want to "help people". I'll have more respect for you if you honestly tell me you're a siren whacker than to lie to me with nonsense about "giving back" to the community.
On the other hand, there are some questions that you probably should not answer honestly. If they ask you what your future plans are, and your future plans are to work here until you can finish the fire academy and get a fire job, I'm not hiring you. And many other EMS employers feel the same way. Employers want a stable workforce. And people who are going to bail on me the first chance they get are not people who are going to contribute to that stability. Most employers are not stupid. They know that nobody goes to work for Joe's Transfer Service looking for a twenty-year career to retire from. But at least try to give them the impression that you aren't intending to walk out on them with no notice in less than a year.