what they told me at my yale interview:
basically you have a conversation with an alumni member from your area, and then they write up their impressions of you - if you are college material, if you are what that school is looking for, etc. then they send their impressions back to the universities. but i think the interview is a very small part of the overall score and most likely if you're borderline it may push you into being admitted.
be talkative but not too much, don't tell any inappropriate stories, dress up a tad (like business casual) and look presentable, listen to what the recruiter is saying. it is just like going into an interview - look them into the eye, sound sure of yourself, and be confident. if you have any questions, make sure they're not answered on the school's website (like how much tuition is) so that they know you're serious about going to their school. above all, be relaxed. if you're nervous they can sense it.
give yourself anywhere from 1-2 hours, preferably up to two hours. mine went about an hour and a half.