Interview is split into two parts, behavioral and case. The case interview comes first. You sit down in a room with an assessor and review the case. You are allowed to ask questions, take notes, make calculations, and see graphs/charts displaying more information. After half an hour, you must present the case to the assessor. (*IMPORTANT: Take notes on every part of case interview, as you're required to submit a write up summarizing your case findings within 48 hours after your interview, and don't want to forget any details). I suggest going online for some sample case interviews to prepare what questions you should be asking and how your write up and analysis should look like.
After your case interview, you go directly to a behavioral interview. The behavioral interview I had, was with a woman from their HR department. She was not very knowledgeable on WMP or their internship programs or incentives, which I thought reflected somewhat poorly on WMP, however she assured me that she had only been with WMP for a few months. Otherwise, I thought the behavioral interview went pretty well.