I would assume your definition of case partner would include both receiving and giving cases.
As for receiving cases, the main way to ensure you are a great partner is to be transparent before the case with the other candidate on your points of strength and weakness and your current level of preparation. This would allow the other person to provide a more suitable type of case and focus on your potential improvement areas.
As for giving cases a great partner would stand out for (1) the case experience provided to the other candidate and (2) the feedback given at the end.
- The ideal case experience should let the other person go through all the main areas in a consulting interview (fit questions, introduction, structure, graph analysis, math, sum up, questions at the end). A great case partner would therefore prepare a case that would try to include most of these elements. Ideally, you may want to use a case with a proper solution already present, in order to make the feedback at point 2 easier.
- The ideal feedback should cover all the main elements seen in the case, and a sum up with the main improvement the candidate should focus on. If you have prepared in advance a good case with a proper solution, you should be able to determine for all the previous areas if the candidate provided a good answer or not.
As for your question How can you adapt to the different prep levels, especially when you just started practising?, I would assume you mean how you could adapt the case you have prepared to people with different preparation. The most important thing would be:
- prepare well in advance the case as for step 1 above, so that even if you have just started, you can be confident your feedback will be appropriate;
- be ready to move on once understood that a candidate is not able to solve a specific section, spending later more time in the feedback explaining what is the right approach for that particular area; in this way you will manage better the time for the interview and make it possible to cover all the main points even with less experienced candidates.
Hope this helps,