If you've looked at previous work or flash or paintings of theirs, maybe let them know why youre excited about that particular person- which is the reason youre comfortable giving control.
"I saw X tattoo in your portfolio and X and X and I really liked those! Im open to whatever youre into right now, I trust you etc" and typically they can feel you out from there and start firing off some ideas.
im sure a quick consult will get the person going in the right direction.