Hi Anonymous,
In most McKinsey-style cases the first question is a kind of question asking to lay down a structure.
If you just know the case prompt and asked clarifying questions without knowing precisely the interviewer's quesiton(s), you will head into a wrong direction (unless the case prompt already included explicitly the question).
Whenever in doubt: Just ask and clarify with the interviewer like in a normal real-life client situation on how he would like to proceed. The interview is not a one-way street!
Hope that helps - if so, please be so kind to give it a thumbs-up with the green upvote button below!
Robert