That’s a good and important question. I am glad you asked.
People with finance background normally do this. This is definitely one of the most useful approaches to profitability cases. However, interviewer in 90% cases gives you a synthesis from financial statements e.g. revenue is decreasing/stagnating, costs steadily increase last 5 years.
You should take into acoount that you still structure case as revenue and costs. And then you can use different options for deep-dives in costs:
- Divide between variable and fixed
- Divide costs according to income statement (your suggestion)
- Divide costs according to value chain
Revenue may be divided into the classification from cash flow statement. But it is not typical for cases, as it heavily depends on a client business.