Consentino's Case In Point states that an interviewer may call your bluff or outrightly say your approach to reaching an assumption or mini conclusion is wrong in order to see how you'll react. So in the case, you either stand your ground or analyse both sides and acqueise depending on the stronger argument. However, when do I decide if, when an interviewer does this, he's actually trying to help me or he's trying to test me?
When to stand your ground in an interview vs following the interviewer's prompt
First let me state that for MBB interviews, this kind of behavior is VERY uncommon! I can say that during my time as interviewer for both McKinsey and BCG, we were actively trained to NOT mislead candidates during cases!
So if you are challenged on an approach, chances are that there are indeed points you should consider. In this case I would clarify with the interviewer where the problem lies (i.e., start recapitulating your basic ingoing assumptions and deductively confirming the logical conclusions regarding relevant elements). Going through your approach elements one by one will then force the interviewer to either explicitly discuss "wrong" elements of your approach or to pinpoint missing elements. It is essentially a structured testing for MECEness. The result of this process will be either a refined approach, or the interviewer needs to admit a bluff (as I said, unlikely at MBB).
To be honest I've never seen anything like that. I recommend assuming by default that the interviewer is trying to help you:
- Since the time of the case is limited (25-35 min), the interviewer will be giving you the tips to help you reach the conclusion faster. It's important to listen to these tips. If you hear this tip - it's important to dig deeper into this issue immediately, saying something like: "That sounds interesting. I would like to investigate this information further and if there is nothing there I will get back to my initial structure"
- Interviewers may be challenging you asking to provide more evidence (arguments) or asking how you would check the particular assumption on a real project. But I've never seen any interviewer saying that you are wrong if you were right.