As a candidate, I've struggled a lot with cost reduction cases since most of the case books don't have them, while you can face these cases on the case interviews. This is especially relevant for McKinsey interviews.
Based on my experience most of the candidates end up by segmenting into fixed and variable costs. Obviously, this structure is quite poor.
My recommendation is to use the process approach which is similar to what consultants usually use on a real project:
- Cost segmentation and prioritization - here you basically try to understand what is the cost structure and what are the biggest cost buckets
- Internal and External Benchmarking and understanding the potential - you compare your costs with competitors, industry benchmarks or internally (Imagine one of your entities having 1 accountant per 100 employees and another 5 accountants per 100 employees)
- Process improvements - in order to cut the costs you need to identify the best processes and scale them across the organization. You should take into account that there are "major process steps" like production, contributing to the output and "supporting process steps" like cleaning. The former are usually optimized with technology or best practices, the latter are usually cut
- Costs & benefits - here you calculate the total impact and the rollout plan
One great advantage - it is really hard to argue with that approach since it's based on real consulting projects.