Informal / conversational
No predetermined questions are asked, in order to remain as open and adaptable as possible to the interviewee's nature and priorities; during the interview, the interviewer "goes with the flow".
The guide approach is intended to ensure that the same general areas of information are collected from each interviewee; this provides more focus than the conversational approach, but still allows a degree of freedom and adaptability in getting information from the interviewee.
Standardized, open-ended interview
Here, the same open-ended questions are asked to all interviewees (an open-ended question is where respondents are free to choose how to answer the question (i.e. they are not "yes" or "no" questions or ones which require a numeric rating etc). This approach facilitates faster interviews that can be more easily analyzed and compared
Closed, fixed-response interview
This is where all the interviewees are asked the same questions and asked to choose answers from among the same set of alternatives. This format is useful for those not practiced in interviewing.