Do you have any tips for thinking out of the box? (e.g. when building a bridge there could be projected utilization but also latent demand later when the bridge has been built, etc.)
This is tricky one to prep for. I find people are either naturally lateral (out of the box) thinkers, or more linear. I tend to be much more linear personally.
With respect to the "Forecast Demand" question, that's less of an "out of the box" question and more of a how do you forecast demand that you currently can not measure question. I had a similar case on forecast demand for cell phones assuming it was 1980, how would you do it.
The approach I came up with at the time (and continue to like) is to find an analogy. For cell phone adoption forecasts, I asked for data on television, radio, fax machine, VCR adoption, looked at the shape of the adoption curve over decades, super-imposed it over the first 3 or 4 years of cell phone data and tried to guesstimate what the adoption (demand) levels would be over the next 30 years.
By the way, she is a very, very sharp person. I hear from a friend who used to work with her at Bain that she is a very fun person to work with. But with me her demeanor was stone cold - no facial expression. I said something right, no reaction. I said something wrong, no reaction.
I understand she did this deliberately to see how you'd react with a tough, expressionless client which I think is fair game. I did pass her interview, but certainly could not tell from my interactions during the interview. Even when I "nailed" the case and she said, "that's exactly what we did" (or something along those lines), it was expressionless too.
The general gist of the interviewer's mentality is that if a client would reasonably do something to you as a consultant, in my book it's fair game to replicate the experience in an interview. So don't get freaked out by this in an interview. If you do, you're likely to get thrown by the same thing from a client. (See more case interview examples and a case interview sample on this site.)
Overall, I've found that if you enjoy the interview experience, you'll probably enjoy the job too. I enjoyed both quite a lot. Most people never quite figure out this point. If you find a case interview to be torture, I can guarantee you that the on-the-job experience will be the same.
I found doing the 60 odd case interviews that I did to be a lot of fun and found the job itself to be a lot of the fun (but ultimately bailed because of the travel and lifestyle -- My second year I spent 48 weeks out of 52 out of town 4 days a week).
I did have a number of my colleagues voluntarily leave McKinsey within the first 6 months. They were very smart people, but just did not enjoy the work. I suspect that they could have figured this out in the interview process.
If you love the interviews, you'll love the work. If you hate the interviews, you will hate the work.