the issue is making sure that your approach is always top-down, it is not about recognizing which case requires detailed analysis vs. broad overview.
What is top-down:
- Lay-out the initial buckets you want to assess without digging deep into each one
- Take 2-3 mins to explain at least 3 sub-segments in each bucket (what do you want to assess, and give 1 example - e.g., To assess market attractiveness, I would like to dig deeper into any relevant trend in customer preferences/habits, such as any potential switch to "greener" alternatives")
- Finish by concluding you overall approach, recommend if it is ok to move the next step or is there something that the interviewer would like you to focus on - say something like "this would summarize the key areas that I would like to explore, I would recommend starting with detailing and analyzing "X", Is that okay? or do you prefer it if I focus on a different area?"
It is safer to stay top-down and not spend too much time on any topic and ask whether the interviewer would like you to dig deeper (and give an example on a potential deep dive that you could do).
Also, maybe the case prep buddies are not managing the interview properly (they should know to tell you, "Ok, let's move to the next topic" rather than let you keep rambling on a specific topic)
I hope this helps