in my experience the main differences are in the fit part of the interview and expected client interaction/communication skills, while the case part per se is not significant different. In particular:
- Fit part: there will be higher expectations on questions on leadership, impact and drive in an interview for Associates compared to Analyst (using McK as reference for the name of the positions). You are supposed to have clear examples in your MBA/working experience showing the skills.
- Client interaction/Communication: as an Associate, you have more client interaction responsibilities than an Analyst; thus they will test more this side during the interview (eg checking how do you react to pressure, how clearly you communicate your thoughts and clarify information, etc)
Both areas may be covered also for Analysts, for Associates the bar will just be higher.
I agree with Vlad that there may be post-MBA candidates not as well prepared as recent graduates. As the case part is similar, for both the type of interviews you can expect a preparation of 150-200 hours starting from zero on your own to be able to receive an offer (less if you are practicing with experienced peers or expert coaching), which is usually more difficult to reach for someone working full time than for a student.