Occasionally I feel impressed by something an actor has had to learn in order to play a role believably. I don't mean frequently seen things like holding a handgun correctly, but some thing a little out of the ordinary.
For instance, Meryl Streep chopping onions in Julie and Julia. She/Julia started out the way most of us do, by slicing the onion and then cutting the slices into little squares. But then she learned to do it the way chefs do, by keeping the tip of the knife down on the chopping board and moving the handle up and down rapidly. I've tried for years to chop onions that way and could never get the hang of it. But Meryl Streep did.