Yin (2009, p107) offers useful insight about the use of an in-depth interview during a case study.
“You can ask key respondents about the facts of a matter as well as their opinions about events. In some situations, you may even ask the interviewee to propose her or his own insights into certain occurrences and may use such propositions as the basis for further inquiry. The “interview” may therefore take place over an extended period of time, not just a single sitting. The interviewee also can suggest other persons for you to interview, as well as other sources of evidence.The more that an interviewee assists in this manner, the more that the role may be considered one of an “informant” rather than a respondent. Key informants are often critical to the success of a case study. Such persons provide the case study investigator with insights into a matter and also can initiate access to corroboratory or contrary sources of evidence. …… Of course, you need to be cautious about becoming overly dependent on a key informant, especially because of the interpersonal influence – frequently subtle – that the informant may have over you. A reasonable way of dealing with this pitfall again is to rely on other sources of evidence to corroborate any insight by such informants and to search for contrary evidence as carefully as possible.”
Reference:
YIN, R. K. 2009. Case study research: Design and methods, Sage publications, INC.