Interviewers will be looking to see how you deal with the unexpected as well as how flexible you are with processing last-minute information. You have been assigned to advise a company with a large Western European market.
You have been assigned to work with a small company that manufactures a popular product. Commercial awareness can be very important to answering market sizing questions. So he should choose to drive his fire engine over the hill. The onions cost have the cost of the carrots. Interviewers want to see that you respect where the limits of available information are, and can quickly determine remedies.
Regardless of the type of case you're presented, there will likely be a few main parameters and several factors that influence those parameters.
Be transparent about your thought process. At some point, the interviewer will ask you to make some simple calculations. Their days are usually spent shuttling from one meeting to another, putting out fires, answering emails and phone calls, and in general, dealing with matters that are more urgent than important.
Some may be even conducted remotely using a video-conferencing app, with the interviewer absent most of the time, and the candidate left alone to work on the case.
It is the process by which you get to your conclusions that interests the interviewer. The message is: practise, practise, practise. Take a moment to think about the case and carefully define the problem being posed.