The quote below is from Hadfield's May 2015 appearance on The James Altucher Show. While he was referring to the difficult job of being an astronaut and training for all of the things that can happen in space, this advice is applicable to gaining expertise in many fields:
So how do you get ready for that? What it all boils down to is an insatiable, permanent necessity for personal competence—to always become better. Because even if you completely master some part of it, some subtle thing is going to change.... All you can do is continually try and improve your understanding of how things work.
...I think if you're not studying something at all times to improve your ability to do things, then I kind of ask: Why not? What's the other thing that you're doing that is more important than getting better at life?..........
Related book: An Astronaut's Guide to Life on Earth: What Going to Space Taught Me About Ingenuity, Determination, and Being Prepared for Anything
Excerpt from the book:
Over the years, I’ve realized that in any new situation, whether it involves an elevator or a rocket ship, you will almost certainly be viewed in one of three ways. As a minus one: actively harmful, someone who creates problems. Or as a zero: your impact is neutral and doesn’t tip the balance one way or the other. Or you’ll be seen as a plus one: someone who actively adds value. Everyone wants to be a plus one, of course. But proclaiming your plus-oneness at the outset almost guarantees you'll be perceived as a minus one, regardless of the skills you bring to the table or how you actually perform. This might seem self-evident, but it can't be, because so many people do it.
...When you have some skills but don't fully understand your environment, there is no way you can be a plus one. At best, you can be a zero. But a zero isn't a bad thing to be. You're competent enough not to create problems or make more work for everyone else. And you have to be competent, and prove to others that you are, before you can be extraordinary. There are no shortcuts, unfortunately.
Even later, when you do understand the environment and can make an outstanding contribution, there's considerable wisdom in practicing humility. If you really are a plus one, people will notice—and they're even more likely to give you credit for it if you're not trying to rub their noses in your greatness.
[H/T Tamas]