An interview is like a very delicate scale and if you know a little bit about the candidate beforehand it's like a big weight on one side of the scale and the interview is useless.For links see Joel 1.
So, before the interview, I avoid anything that might give me some preconceived notions about the candidate. Because, if I think that someone is smart before she walks into the room, just because she has a Ph.D. from MIT, nothing she can say in 1 hour is going to overcome that prejudice. And if I think she is a bozo, nothing she can say will overcome that initial impression.
Once, right before an interview, a recruiter came into my office. "You're going to love this guy," she said. Boy, did this make me mad. What I should have said was, "Well, if you're so sure I'm going to love him, why don't you just hire him instead of wasting my time going through this interview."
But I was young and naïve, so I interviewed him. When he said not-so-smart things, I thought to myself, "Gee, must be the exception that proves the rule." I looked at everything he said through rose-colored glasses. I wound up saying Hire even though he was a crappy candidate. You know what? Everybody else who interviewed him said No Hire.
So, don't listen to recruiters; don't ask around about the person before you interview her; and never talk to the other interviewers about the candidate until you've both made your decisions independently. It's the scientific method.
Joel On Interviewing 4: How To Avoid Bias