Yea, I agree--have an idea of how you would respond to most questions, but definitely don't rehearse or memorize answers. These people interview a lot, and they can smell BS a mile away. Also, don't give run-of-the-mill answers--aim for distinctiveness without being controversial. If they ask why you want to be a doctor, don't respond with a diatribe about your desire to cure disease and wipe out human suffering, even if it is true. Be a little witty with your remarks, but don't make jokes--just show them you are a real person, and not a pre-med drone. I never got asked about health policy or public health issues, partly because every interview was exactly the same for me--senior thesis talk, and then about my volunteering--and partly because I steered the covnersation to waht I wanted to highlight. Know your strengths and your weaknesses, and guide the topics towards the former. This is all common sense--likewise, just be yourself. If you are stupid or dull, nothing you do will make them think otherwise; but if you are charismatic and will be a great doctor, you'll be hard pressed to screw up an interview--they are almost universally very laid back.