You’re not going to find the answer here (arf), but I love this article on what it takes to become an expert. This point I find particularly interesting:
tacit knowledge: automatic, unexpressed knowledge that provides context for people, places, ideas, and experiences. Tacit knowledge is not easily shared. As Polanyi said: “We know more than we can tell.”
I like this too, especially the point that managers become great if they are self aware - they know their weaknesses and they make sure someone is covering their back.