My postdoc is learning a little bit about what its like to be a PI. He's got three different papers he's working on, including one last one from his PhD. He has a fellowship proposal to write, on a topic that is a bit distant from what I do, using the same techniques, but answering different q's. Its a good start towards independence, but it means more new literature for him. (He picked and explored the topic - his choice). He's got experimental commitments to my R01 which pays his salary. He doesn't complain (at least to me), but he has admitted to being a bit overwhelmed.
But when I asked about postponing a set of experiments, experiments that will be arduous and time consuming, he immediately started giving me reasons why we shouldn't. Because the work in my lab - whole animal behavior/biomechanics/neurophysiology - requires many pairs of hands (a single experiment is a minimum of three bodies, but 4 works much better), his buy-in important. His reasoning about this was good. It certainly would be better for him, short-term, to postpone the experiments and work on more pubs. He could probably get another 2-3 from data we already have.
Some of the differences between a senior postdoc and a junior faculty are subtle and along a continuous axis of scientific maturity. If you're running your own project as a postdoc and taking results to the PI once a week, then running a project on your own in a jr faculty lab is often only about available resources. But one of my favorite past posts was "what do you own?". Its an old post, but here's the nut:
When you become a postdoc you own the project. Maybe you own part of a grad student’s career – because what happens to them reflects back on you in ways that the work of an undergrad doesn’t.
When you become a TT person (or sometimes, in some very big labs, a senior postdoc fellow, who is figuring out a non-TT career), you own the lab or your part of the lab. You all of a sudden own the careers of a technician, and any trainees you’ve got. What they do reflects on you. And what’s more you own your career, in a way that wasn’t really so obvious when you were a postdoc and you just owned a project.
When one is at the point of leaving a postdoc, one is most intensely, and rightly so, concerned with owning one's career. With your first job, the goal posts didn't just move back, but they moved to a different golf course, with new sand traps and ponds and a whole different crowd of people waiting to see What You Are Going To Do.
One sign of professional maturity, or leadership if you will, is being able to take a step back from the immediate and intense concerns to look at the bigger picture. Of course its just one sign, and there are 16 other ways to fuck up.