I have been working at Bain & Co for ~2.5 years, so can help you with this question. I did my interviews in the Italian office (Milan), but I believe the approach for the first interview will not vary that much.
In your first interview you will meet 2 managers/ Principals from Bain and each of them will drive you through a business case.
My personal advice, as Bain people are on average very approachable and friendly, try to act as friendly and relaxed as possible (I know it is hard!).
In terms of approach and hints to ace the case, see below a few points:
- Make sure you take some time after the business case has been defined for you by the Manager to set a logic and solid structure; how you think and your logic are things which are pretty important in Bain
- Do not panic if you do not nail the case at first, what the manager wants to test is your ability to think about tings in a logic way and drive logic conclusions. If you do not nail the case at first, try to come up with multiple options which make sense and do not stop/ panic
- In terms of menthal reasoning and calcs, naturally you have to prepare in advance; however, i found that Bain pays more attention to the overall structure and logic in developing the case from the candidate vs.being able to do crazy calcs. In my experience, they always allowed me to round numbers to make calculations easier, whilst McKinsey is very focused on menthal math
Hope this is helpful and that the interview goes well! I remember some of the business cases done in Bain interviews, so let me know if you need any help.