The way to approach this, especially if you have some time before interviews, is to:
1) Read about the content area you know well: This way you'll be able to speak even better about your background, be it in heavy industry or consumer goods or FS. You'll also understand more about how McKinsey approaches 'your' field, and be able to give good, layered responses.
2) Read reports/ articles on the geography you're applying for: Say you're keen on Spain, read all the reports McK has published to do with Spain, so you understand not just from (1) content focus, but also (2) a more geography focus and more about what McK does in your home market.
3) Read what interests you: Lastly, read for fun. Pick out as many articles or reports you can that speak to you, interest you, and make notes so you can flick through your brief notes prior to the interview.