I agree to other experts that you have to be prepared to "normal" MBB interview.
However, I would suggest to pay a lot of attention on the structure and organization of your sheet of paper, where you write the business case. BCG collects it to check the layout of your notes (you should aim to draft like a ppt slide). In addition, BCG asks more difficult calculation with less approx and really like structure thoughts (more than Bain for istance).
Of course, all the interviews are different, but these are some of the peculiarities of BCG.