Be honest and friendly. Smile a lot. Get busy , and ask your questions. Do not ever appear to be wasting their time. Listen actively .
Eye contact is vitally important.
Avoid confrontation but not conflict. Follow up each question with another question on the same subject. It is rarely the case that a subject is adequately exhausted with one question. You may need to ask the same question in several ways, or diligently press for more detail to really get a good answer. If the interviewee strays from the subject, try not to interrupt, but do not hesitate to ask them to talk more about your specific question.