It depends on the context.
You may be asked to calculate the ROI or payback period on a given capital expenditure (e.g., Client is spending $100 in CapEx on a new machine that will allow for $10 in savings every year); in this case you should think of the capital expenditure as the upfront investment cost.
Alternatively, your client may be acquiring a company for $100 that generates $10 in EBITDA and requires $5 in capital expenditures (sometimes referred to as "maintenance CapEx" but not necessarily) every year; in this case you'll need to subtract the CapEx ($5) from the EBITDA ($10) in order to get to the free cash flow that the $100 company produces.
Hope this makes sense!
When in doubt, I would just explicitly clarfiy with your interviewer (1) what is an upfront, one-time cost, (2) what is an ongoing variable cost, and (3) what is an ongoing fixed cost. In this way you can avoid any confusion about what's meant by CapEx in the context of the case.
Good luck with your interviews!