Just a random tidbit that I thought some people here might find interesting.
I was at an Iowa meet and greet, and got to chat with Pete for a moment (Pete Buttigieg is a democratic candidate for President who has worked at McKinsey). I asked him for McKinsey case interview tips, and he basically said the following:
"Use the resources and books they recommend, because those are really useful. When they ask a question, they're looking for how you approach it and break it down – when they ask, there isn't one thing that they're trying to nitpick. Explain how you think about breaking up the problem."
I asked if he drew out an issue tree on a notepad in his interview, and he said (as close as I remember)
"It's not necessary, you can if you want to. The structures and such can be useful but it's more just about how you approach your analysis and problem solve. It's also helpful if you have a friend you can practice and go through cases with, because a lot of times they can give you good insight or feedback."