They grill you by asking extremely open-ended questions and then super detailed questions so that you will trip up or crack under pressure. They are wanting to see how you react. One guy interviewed me in a small room with couches and later a woman came in and spoke to me. Then another guy from a different department came in. They will do a few mistakes here and there that will look like mistakes, but really they want to see how you would react in the moment. It's all understandable though because they're a high pressure environment.