This is dependent on the context of the interview, and will really vary from situation to situation. For example, in an MMI situation it is unlikely that it would be appropriate to ask a question to the examiner - it is not a conversational format. Conversely, it is recommended during an oxbridge interview to ask questions to further your understanding. Interview tutoring with our medical students is an excellent way to understand the nuances of the situations and help you get to grips with what exactly you should be doing in your interviews.