Just to chime in, for me it's very much about communication and semantics. The more the director can communicate desired aspects/characteristics of what is needed, the more I can nail down their vision.
As for finding the right artist for you - Again communication. Pick a couple you like and open up a dialogue, "hey, I'm working on this. Is this the sort of art style you do? Would you be interested in a project like this? What would be your approach? Speaking with various potential artists also gives you a "get-to-know-you" buffer, and you can better evaluate if this is a person you want to work with.