An Eagle Board is basically a summative evaluation of the candidate. Everything's on the table: the boy's Scouting experience; his family and life experience; his personal values; the project he led; his future career plans; even what team he's backing in the playoffs. An Eagle Board can wander all over the place, and when the interview is concluded, the Scout retires so that the Board can discuss him before voting on his application.
A unanimous vote is required to award the Eagle Rank. This is mostly for the sake of the Scout, for whom the words "it is the unanimous decision of this Board" carry some extra affirmative weight. And the only ways in which one could actually garner a negative vote or two would be to really, really screw up -- like, LIE to the Board, or be so arrogant that you come off as insulting, or be so scared that you freeze up and can't communicate at all.
In the end, all that is asked is that the young man be able to represent himself well to a group of adults -- to have a candid conversation about Life, the Universe, and Everything with people who want to know Who You Are. It is a friendly conversation, not a hostile one -- but it does intend to probe a bit. And it ends (at least, I've always seen it end) in congratulations. The Scout is left feeling affirmed and the adults, proud.
I'm glad I've had the opportunity to lead these things. I've seen a lot of great young men and got to share in their achievement.