Another aspect that is very important (and usually violated in Case Coaching books) is the principle of first isolating the numerical problem driver, before asking qualitative question. Never start your analysis with asking qualitative questions ("First I would like to get a general understanding of the market development" and such phrases)! This is practically the very definition of "boiling the ocean", i.e., working in an extremely inefficient way.
Instead, you should first seek to narrow down the area which you even need to qualitatively understand! And this can be done very quickly by doing a numerical analysis as described above. Once you know where the problem comes from (the numerical problem driver), THEN you can start to understand the qualitative reasons that underlie the negative development of this driver.
This analysis will be far more focused and concrete than if you would have tried to discuss qualitative reasons at the start.