I'm not familiar with McK Mock Interview but I can give you my experience being invited to an informal conversation with a principal at BCG.
This is a great way to know more about the firm and basically start the recruiting process.
Expect regular fit questions: "so tell me about yourself?", "why do you want to come to consulting?".
It's important that you prepare some questions to ask him as well, and it is crucial that in your questions you actually show that you did your homework on the Firm (ex: McK recently opened a new digital capability center in Aachen, what other projects does McK has in the pipeline for the future in order to prepare itself for the changes in the market?)
I'm not sure if you are going to receive any case but just in case prepare that as well so you are ready for anything.
Other than that just have fun and don't forget even if they appear to be casual they will be evaluating you.
Hope this helps!