I always expect the artist to have their own style. I give a moderate level of instruction and encourage the artist to be creative. I usually like to see a sketch or lines before color, in order to suggest changes.
Mostly depends on the commission. For example my 'We Can Do It' pic with my heroine Miss America I was pretty specific that it should have that pose and we see all of Miss A. However for another pic I had I just said, she's flying. It all depends on what I'm looking for.
I always allow the artist to use their style though.
Well my theory is always that you hire a person (in any field) to do the job in their way. So when hiring an artist I look through the portfolio to see if I like their style before I even talk to them.
Than what I would love to do is discuss the concept with the artist and let them do what you think up with it. I want to approve sketches. This way I can make sure it is something the character would do, especially if it is my OC and make sure any notes about outfit design are discussed. But after all the points are discussed and concept is approved I just want to wait till the final.