A.) your head
B.) your calmness
It's funny how people are always wanting to know how to prepare for an interview. If you think about it, an interview is a discussion to see if you fit with the group you are inteviewing with. I have seen many highly qualified phd come and go for interviews with the current pharma company I am with. The ones that can think through a question and provide a personable and precise answer almost alway get the job. The ones who lose their heads and take 20 min. to answer, when a 2 second answer would work, are wasting time to think of an answer. Thus the reccomendation of the above two things. They already somewhat like you because they offered an interview. Now they are seeing if you really fit and if you were telling the truth about your credentials.
DR