A clear idea of what your mentor expects from you and what you expect from your mentor can make for a very symbiotic mentor-trainee relationship. I've seen trainees get read the riot act for not living up to expectations that they didn't know about until later in the game. For instance, a PI lit up a 2nd year postdoc for not working enough in the lab since they started. Whether this is right or wrong is not germane, um should they be having this conversation nearly two years down the road. After a few months, maybe this talk should have taken place. And on the other foot, I've seen graduating PhD whine to their PI's that they didn't have enough experience in scientific writing and grantsmanship. A little late for that one too.
A healthy relationship requires both the mentor and trainee to bring up their concerns in a timely fashion and not to mention that if you have the problem, the onus is on you to bring it up. Its not going to bring itself up. Some of my biggest growths as a scientist have been when the boss has called me into the office and told me that I'm not living up to my end of the bargain and that I need to correct whatever deficiency that I have. Avoiding these conversations is like allowing a nasty wound to fester.