Because some questions may be reused from year to year, and because you shouldn't be obsessing over this, I won't list all the questions I was asked. Here, however, is a sampling:

My favorite: If you had to choose one or the other, would you turn off friction or gravity?

My immediate response was "Am I just going for coolness here, or what?" I then proceeded to flounder. First I was worried about the formation of stars and planets; I didn't want to turn off fusion by turning off gravity, or prevent any solar system from having formed. Then, when I was given a hypothetical planet to play with, without stars in the pictures, I worried about people falling off, avoiding the question of which force is more important in everyday life. I think this may have exasperated the judges somewhat. But it's an awesome question.

My least favorite: What's the difference between a cancer cell and a normal one?

I picked this as my least favorite simply because I didn't know. There's really nothing wrong with it otherwise.

A "typical" question: How can you tell if a star has planets orbiting around it?

I was proud of actually being able to figure a few ideas out. As I learned in Ay1 section recently during Caltech Prefrosh Weekend, there are three generally accepted methods: looking for redshift/blueshift (most reliable, and most likely to work); looking for wobble (a bit harder to measure, and you have to be lucky); and looking for a shadow as the planet passes (gives a lot of information, but you have to be really lucky).

So how can you prepare for the interviews? Well, a basic knowledge of chemistry, math, and physics wouldn't hurt, but you'd have to do an awful lot of studying to really improve your chances of knowing one extra question. Being well-informed about current events (particularly in science) is a good idea, but again, is unlikely to really make a difference in how you do. We were advised by the media training guy to read the newspaper before interviews and thus most people did so religiously, "cramming" current events in the waiting room. Although we were thus very well-informed for a few days, I personally didn't come across any questions for which this would have been even remotely helpful!

Essentially, the judges are really good at coming up with questions that probe how you think rather than what you know, so the best practice is to think about things and perhaps to practice thinking out loud even when you're unsure of yourself.

Do have a pet theory or idea that you love to talk about ready in case you get an open-ended question you can steer in your favor. You may get a question so broad you can take it anywhere you want--if you have a destination in mind, it's easier to check that the question sort of fits and go with it than it is to actually try to interpret the question. Fine, fine, an example. One of my first interview questions was "Tell me a bit about global change." It turned out that the judge meant "global warming," but I talked about viewing evolution at a societal rather than organismal level for a good five minutes before he said so.

Overall, perhaps the best thing to do is to give yourself permission not to prepare. Relax and have fun with the interviews. Some of the questions really are ones you'll think about for weeks.