I was just asked to explain my research to them. Strangely, I was not grilled very heavily in-person. The phone questions were more like "Do you have experience with <this skill>?", which is basically a yes or no question. I was flown up to Milwaukee for an interview in-person, and was interviewed by six different team members. They wanted me to discuss my research, and just asked me various questions about myself, which I think was probing for "Am I a nice guy?" I was treated to lunch, which was quite tasty, and shown around the building. They paid for my hotel room for the night at a nice downtown hotel, as well as all of my food and airfare. One interviewer did grill me on my knowledge in a particular subject, because he was skeptical as to whether or not I would know the material. I was able to discuss it with him, and he was put at ease.