Well done on securing the interview! If the topic of the case study isn’t specified, then it likely is relating to a business engagement, or some sort of article which requires you to form opinions on either general business/ economic trends, or specific policy questions. Having a good knowledge of basic economic concepts, business drivers, and current events, will enable you to properly discuss on any given area. That said, another way to gauge the topic of the case study more specifically, is to look at the firm’s core practice area(s). In my experience, the transactional-focused law firms tended to sway towards their case studies being on M&A (discussing the viability of a proposed deal) and vise versa for any other practice area that is core at a given firm. Having good knowledge of what’s going on in that industry helps you navigate such scenarios. Another thing is that despite this being a likely occurrence, focusing on other areas of the firm‘s practice, and then weaving this into your discussion, shows an all-rounded candidate and a wider understanding of their business. For example, if it’s an M&A case study, showing an awareness of the firm’s disputes practice in dealing with any litigation-related aspects of the target company’s operations, shows you appreciate considering the broader picture. In short, there is always something to link it to. If a situation arises where you are unsure of a topic or point, feel free to ask your interviewers to clarify - this is better than waffling and gives you time to make an informed, educated, conveyance of your points.
Best of luck in the interview - we’re rooting for you!