I got lucky because Harvard only sends people from the Admissions Committee, and this guy in particular was not your typical Harvard guy. It turns out half the year he works as a Napa Valley wine grower. He was a Harvard Business School graduate who got sick of working in consulting and so he spends six months in Napa writing wine reviews, and to make sure he has enough money he goes and works the other six months for the Harvard Admissions Committee. He's a part time worker. That makes him a little more relaxed than all the other recruiters.
Right when I walked in he told me to loosen my tie, relax, put my feet up. The interview was really relaxed. Honestly, I think when it comes time for Harvard, and I can't say this for every school, they've already validated that you're smart. They just want to make sure that you aren't a terrible person.
I've heard that 6 out of 10 people who interview for Harvard Business School get in. Of the four that don't, you either don't get in because A) you can't speak English well enough, B) you're extremely arrogant, or C) you're extremely dry. As long as you show you aren't one of those three things, you have a pretty good shot of getting in, generally.