The final round is trickier because it is not as strcutred as the earlier ones. Once you pass the initial rounds, it is clear that you are considered capable enough to be a consultant in the firm. Now, the partners are looking to see if you would do well in the firm and on their teams. THe following changes are what to prepare for :
There is now a much larger emphasis on PEI. The aim here is to see whether you are client ready and if you will be a good fit at the firm. What they typically look for is - your style of communication, your listening skills, how open are you to coaching and feedback and what sort of energy you bring to the table. Make sure you up your PEI game by practicing 5-6 relevant stories and having a clear and structured method of communicating them. You will also need to practice listening skills and being tailored in your answers.
An additional element here is also to come across as charming/personable. You want to have a good conversation with the interviewer and not just focus on walking through your stories
Cases in the final round are typically less structured. Partrners use cases here to see how you would approach a problem they have come across and want to test you on your ability to clearly structure your thoughts, perform quick analysis and come up with reasonable conclusions/next steps.
This is also where they test you for things that might have come up as red-flags in round 1, so if you felt you did not perform well in structuring earlier make sure you really structure well in this round
This is also the time where you may be stress tested - this includes curve-ball questions and things such as creating high pressure environments for you by for example ignoring you or asking you to answer in a short amount of time etc.
Hope this helps and all the best!