I strongly prefer interviewee-led cases. These are much more challenging because rather than give you structure, I ask very open ended questions and you are expected to drive with your own hypotheses, assumptions, etc. However, to be a successful consultant means to not shy away from this. Consequently, I recommend seeking my help only when you have practiced cases and are already relatively prepared. That said, if you are not or stumbling, I am not the harsh-silent type, and will try to guide you to some kind of answer, using where you end up to assess your capabilities. I also tend to give you harder cases, my philosophy is that if you can do really well on challenging problems, the easy ones should be very easy! I incorporate graphs and charts that I think you will see, twists and turns that might be more rare, all to help you better prepare for the case interview.