For now I only did the first round, so this information is only about this round. They do a series of group interviews with roughly 40 people each at each session. First you get some breakfast with the whole group and then a company presentation. Afterwards you get to read an information leaflet on the case you'll be doing, and you will be divided into groups of 4. In these smaller groups you get about 30 minutes to discuss the case and come up with a presentation. There's essentially nothing you can do to prepare the case interview, and there is no need to.