This is going to be very firm-dependent! In general, to second Moritz's point, I very much agree that, unless the firm specifically told you otherwise, you cannot bank on the case being FS-related.
You haven't given too much detail about the company, what stage of the interview process you are at, or how much prep you have done so far: if you let us know some more details, then we can be more specific with advice.
In general I would say:
1. Get as much information as you can about the company, their interview approach, the partner, and their interests. There might well be some information online about how interviews typically go at this company. At a partner interview, they frequently (though not always) will bring up cases from their personal work experience, so it's helpful to know what their background is in.
2. If there's any chance that you may have to do a case or a behavioral interview, definitely prep for those. A lot of the recruiting process is unpredictable, but this would be one area where you can set yourself up for success. There are lots of guides on Preplounge as to how to begin approaching case/fit prep (if you haven't done so yet), and if you are interested in working with a coach, feel free to reach out!