Each school will be a little different. Have you had any MD interviews? In my experience, its pretty similar. But they also ask something about osteopathic medicine.
At GA-PCOM, they explained to me why DO school might be better for me personally, given my background and goals. I tended to agree with everything they said.
I didn't prepare for my interviews. I just went in and answered honestly. I think this cut down on me trying to guess what they wanted to hear, and I think I performed well as a result.