My department conducts interviews with a "long list" of applicants that we are considering admitting. The way that we do it is:
1. In the first half of the interview, we (one of the faculty in the department) typically ask the applicant some questions based on their application. So if we see that the applicant is the co-author on a manuscript, we usually ask them to explain their contribution to the paper, what challenges they faced and how they overcame them, etc. If the applicant has teaching/tutoring experience, wrote or is writing a thesis, worked as a Research Assistant or did an REU, or mentioned some research interests in their statement of purpose, then we often ask about that. We don't "quiz" the applicant about their knowledge of their stated research interests -- it's more like, how did you become interested in this area? If there was one or two semesters of weaker grades, the applicant also has an opportunity to explain this.
2. In the second half of the interview, we ask the applicant if they have any questions for us, and we answer their questions to the best of our ability. This is probably one of the most important parts of the interview, as it conveys your interest in the program and shows that you have done some research about the program. If an applicant does not have any/a lot of questions or if they say something that comes across as a "red flag" (like the applicant confusing our program with a different one -- it's happened before!), then it might give us pause and cause them to be rated down a little bit. But if the interviewee asks very thoughtful questions, then it can definitely help improve their rating/ranking.
After the interviews are conducted, the admissions committee meets again and re-scores/re-ranks all the applicants, and then the top [x] ranked applicants are sent first-round offers. Everyone else on the long list (typically about half of the long list) is kept on the waiting list and has to wait to see if spots open up.
I don't know if there is anyone who also get this interview invitation. But I figured people may find this useful:
It was a 20 minutes Skype interview and the questions were standard. They first asked me some general questions:
1. Introduce yourself
2. Why Duke is a good fit?
3. What's your career goal?
And then they asked me some details about the experience that I wrote on my CV, and what I found the most enjoyable doing research. They told me that I will hear from them in February and if admitted, there will be a visit day.
Good luck!
