Patching and Plastering - Tiling A Shower Stall ?

Hello:

We have a shower stall that's tiled half way up. House is about 25 yrs old.
Will be calling in a contractor to tile the rest of it, and also the ceiling.

Thought I'd ask here about what to ask him, as I'm not really sure ?

Also have these specific questions, please:

a. The ceiling and upper wall are flaking a bit, and are probably nothing but the typical plaster drywall.

Do they tile right over this, or is it better to totally remove this drywall, and re-install something more waterproof, hopefully, like a concrete-mix type of wallboard (I undoubtedly have the name of this product wrong; but that's what it was termed about 20 years ago) ?

If so, what product(s) should I request that he use ?

b. What else should I be asking him ?

Much thanks,
Bob

I don't think it's absolutley necessary to put durock or hardibacker (cement board) above. But if you want to it will be fine. I don't think that anything other than regular drywall is under the original tile. Of course thats just a guess.