In the preparation journey, it's good to combine training with other candidates and consultant /coaches. The first is a good way to build some experience by solving multiple cases with someone that has time to offer (since he is in the same process). The second is mandatory when it comes to prepare for real situation and experience what will be the level of expectation.
As you mentionned it, training with other candidates has its limits, most of the time due to lack of concrete experience of the interviewer. the most common mistakes :
- not adapting case narrative to candidate answers.
- providing too much / not enough insights to solve the case.
- being too soft without enough time pressure (which will not help for the real situation)
- Mis-judgement on overall performance
- Weak / limited feedback not covering all dimensions that will be evaluated in real situation
So this is for the most common mistakes. On the other hand, keep in mind that acting as interviewer yourself from time to time may help you to understand how it feels from the other side, and realizing the mistake of other candidate will help you to improve your own performance.
Hope this helps