Green interviewers sometimes fall into the trap of permitting the interview become “free form”, spending various amounts of time on a number of questions, basing follow-up questions on how the candidates reply.
This can result in a candidate getting control of the interview and driving you where he or she wishes to go, rather than where you are able get the information you actually need.
Prepare a list of questions in advance, based on the information you require, and use it as a guide during the interview. Place each question on a separate sheet of paper and get ready one set for each candidate.