Not if you mean plastic behind the drywall. I would use a PVA primer though.2) Should I use a vapor barrier on the 'internal' walls? The exterior wall already has it in place.
I don't know that they require it, but I like to have it flat to start with. Tiling is easier if things are flat I would tape any tapered joints and use a vertical sheet for the "end" of the L without tapered edges. I don't like to build up the corners as a normal drywall corner though (because they aren't flat). FWIW, The demos that I've seen them do at trade shows use vertical drywall and with no taping done. At least some of your walls are longer than 4' and you might get sick of doing vertical seams...3) In the shower when the sheetrock is in place, before the kerdi system is installed, is it necessary to mud & tape?