Paying attention to behaviour rather than to what people say they will do. On the floor, the gap between the two is visible within about a minute, and it is where most of the useful information is.
Earning trust before offering expertise. Telling someone the cheaper option is the right one, or that we do not have what they need, is what makes everything said afterwards worth listening to. It works the same way with suppliers.
Holding three things at once with no playbook: what the customer needs, what an external partner can realistically deliver, and what the commercial reality allows. Nobody hands you the answer to that, and it usually cannot be optimised for all three. Most of the work is deciding which one gives.
I am comfortable saying I do not know yet, and then going and finding out. Nearly everything I can do now, I could not do two years ago.