l'evian I remain unconvinced about the common sense so far, but you seem a fairly balanced and normal individual so here goes..
First, as Rita advised, go familiarise yourself with case study questions - you will get them alot in MC interviews, so you may as well get practicing. Key to getting through them - much like a math question - is show your workings/thinking, how you go to a solution (it doesnt actually matter if the solution is wrong that much). Expect to justify your answers. Case study type questions can be thrown in at any time, not just as a formal part of a process.
They will be asking you about past behaviour to show how you will behave in future, and for evidence of certain skills. If you dont have relevant experience (which would be unlikely as a grad entry) you will need to demonstrate those skills from your extra curriculars, part time work etc. They won't be asking you for recommendations on migrating HBOS to x core platform in payments, but will be asking for examples of leadership, handling difficult situations etc. Think through your CV and try to make sure you are not using the same example to answer more than 2 questions at the most. At this point its all about transferable skills - you dont yet have the hard knowledge so they will focus on your raw potential.
And SMILE. Easy to forget in an interview when you are nervous, but good eye contact, smile, be responsive, mirror the body language of your interviewer, all goes a long long way. For the first year of your working life in an MC you are basically a cost centre, so whether you come across as someone they would like to have around will probably tip the balance one way or another.