Another cause for the respondent's hesitation might be the perceptions and concerns as to what the interviewer might be thinking about the answers to the question. Hence there are "face" issues at work in interviews on both sides (interviewer and interviewee). See, for example, Goffman's work on face in _Interaction ritual: Essays on face-to-face behavior_. Best wishes, ~Jenny <snip snip>
People hesitate for many reasons -- they are lying, they are unsure of what they are saying, they are not sure whether they should reveal something, they are searching for the words that say what they mean, they have never articulated the answer to a question someone (such as an interviewer) asks. The problem is is figuring out which reason applies in a given situation.