We have found it important to adhere to the three-interview structure. Each interview serves a purpose both by itself and within the series. Sometimes, in the first interview, a participant may start to tell an interesting story about his or her present work situation; but that is the focus of the second interview. It is tempting, because the information may be interesting, to pursue the participant’s lead and forsake the structure of the interview. To do so, however, can erode the focus of each interview and the interviewer’s sense of purpose. Each interview comprises a multitude of decisions that the interviewer must make. The open-ended, in-depth inquiry is best carried out in a structure that allows both the participant and the interviewer to maintain a sense of the focus of each interview in the series.
Furthermore, each interview provides a foundation of detail that helps illumine the next. Taking advantage of the interactive and cumulative nature of the sequence of the interviews requires that interviewers adhere to the purpose of each. There is a logic to the interviews, and to lose control of their direction is to lose the power of that logic and the benefit from it. Therefore, in the process of conducting the three interviews, the interviewer must maintain a delicate balance between providing enough openness for the participants to tell their stories and enough focus to allow the interview structure to work. (See McCracken, 1988, p. 22, for further discussion of this delicate balance.)
Source: Seidman Irving (2006), Interviewing As Qualitative Research: A Guide for Researchers in Education And the Social Sciences, Teachers College Press; 3rd edition.