Be comfortably prepared with:
1) Why Consulting
2) Why McKinsey
3) Why this role
You need to have your story ready, and convince them you are passionate + serious about joining McKinsey AND that you are an impressive candidate that they would want.
Furthermore, make sure you know the company. I.e. What the service ops practice does, the Partners that run it, what some of their recent insights/projects have been, what the job looks like day-to-day, etc.
You can also take the opportunity to ask some quetions specific to their office, but make sure you're not asking silly questions that you should have found out before from some research! Demonstrate your interest, competence, and likeability.