From getting that exact question a lot, I say they "think" they know. If they do "know" they have like 1-2 shots in mind.
What I usually do, is ask them to bring samples with them, other photographs, magazines clippings, internet pictures, etc. You'll end up with 2-3 shots, the rest of the sitting is up to you, you have to apply your style, see what works for their body type.
Ok so funny story, I had a 5'6" woman ask me to reproduce an advertisement, she wanted it for her husband's birthday. Sadly the subject of the photo was not "human", she would have been 7'4" tall in one leg and 5'2" in the other. I worked with her and got a shot that she liked just as much.