Hi there,
My board wasnt particularly grilling at all, (I walked out smiling and slightly confused, because I was smiling), relatively, after the very busy two weeks of flying about and spending your spare time doing your "secondary duty" (especially in the last few days, unless you get an easy one) it was a relief to do something you couldn't completely prepare for. They look at who you are, you can't go in there and say you have outstanding maturity and leadership levels if you don't. They want to see what kind of person you are, and if your the one they want to spend some serious money on.
As others have written:
HONESTY and INTEGRITY work well. Essentially that is what you need, and some confidence. Approach the group activities in a positive way. I enjoyed my board interview, which I did not expect.
Good Luck to everyone in the process.