The MBB experience has trained me to hold interview with an 80-20 approach.
I generally suggest at least 3 meetings:
1) to review the CV and to fully understand the MBB expectations in term of interpersonal and presentation skills, followed by a mini business-case that I propose to all candidates in order to define key weaknesses and spikes adapting the following 2 sessions to personal and specific needs
2) Generally I start with a top-down business-case (hypothesis/assumptions driven approach)
3) I conclude with a more detailed business-case to emphasise focus on analytics and structure
