Try not to stress about interviews too much (and this bit of information makes me a huge hypocrite, because 7-8 months ago this is precisely what I was doing every second of every day). Seriously, I actually enjoyed most of my interviews.
And along the lines of making people feel better, here are some of my unhappy interview moments:
--One of the interviewers was (if I'm remembering correctly) a lab animal vet and I worked in the path lab of a research facility for a while. She asked me what PPE we used during NHP necropsies (which I knew just fine) and then she asked me what pathogen we're really concerned about with NHPs. Which is not a hard question and I knew the answer. But for some *****ic reason when I try to answer that question I always mix up Hepatitis B and Herpes B. It's just when I say them out loud I have a hard time remembering which one is which (I think it's like spelling out loud...not my forte). Anyway, so I knew I might say the wrong one, took a second to think about it, and came up with Hepatitis B virus. She looks at me weirdly and says, "Do you mean Herpes B virus." I'm sure I went 17 shades of red. Gah. Open mouth, insert foot.
--At a different interview, I got asked about prevention measures for WNV. In my defense, I had just been asked a whole bunch of questions about avian influenza pandemic preparedness (i.e. human public health stuff) so I was thinking about human prevention. So I start talking about vector control (fine), and education about wearing long pants/long sleeve shirts, bug spray, etc. etc. Then they guy who asked the question looks at me and says, "Who cares about people, tell me about prevention for horses." Yea, I definitely felt like an idiot for a long time after that.
But in the end, I got into both of those schools. And I didn't get asked any totally out of left field questions, though that is one of the major things I was super paranoid about in the months leading up to interviews. So really, try to relax, be confident, admit if you don't know something, and don't beat yourself up too much if you think you messed up on certain questions. I think the overall feel of the interview (for both you and your interviewers) is more important than whether you might have flubbed one or two questions.