Structured Interview - standardized questions read out by the interviewer according to an interview schedule [i.e., pre-set specific interview questions]. Answers may be closed format [i.e., yes/no, true/false, multiple choice]. -- Quantitative
Unstructured Interview - a flexible format, usually based on a question guide but where the format remains the choice of the interviewer, who can allow the interview to "ramble" in order to get insights into the attitudes of the interviewee. No closed format questions. -- Qualitative
Semi-structured Interview - one that contains structured and unstructured sections with standardized and open type questions. -- Qualitative and Quantitative
Adapted from Nicholas Walliman, Research Methods: The Basics
Four Principles of Interviewing
1. Prepare carefully, familiarizing yourself with as much background as possible.
2. Establish a relationship with the interviewee conducive to obtaining information.
3. Ask questions that are relevant to the interviewee and that induce him/her to talk.