Interviews between a respondent and an interviewer in which the interviewer asks probing questions to really understand the participant’s motivations by using non directive techniques. Depth Interviews are conducted one-on-one, and generally last between 30 minutes and one hour. They are an in depth analysis of personal opinions, beliefs, and values. In Market Research Depth Interview may be useful in revealing covert  issues. Commonly they are unregulated, differing from survey interviews in which identical questions are asked to all interviewees.

See also:
Market Research
Respondent
Data Collection
Focus Groups