The bathroom wall begins with a choice made for what happens on top of it. Moisture-resistant board or standard drywall — that decision is not about the bathroom as a room, but about the finish that will cover the board. Paint does not demand resistance; tile and waterproofing do not care what is underneath either, so long as the board is stable. But the way you finish the surface against the board — the taping, the joints, the corners — that changes entirely depending on whether the wall is going to be painted or tiled.
A wall under paint needs every imperfection at the board joints faired smooth. The tape is covered completely, the compound fills the gaps and then gets sanded to nothing, so the wall reads as one seamless surface. A wall under tile does not need that. The finish level drops dramatically. The tape does not have to disappear; the joints do not have to be perfect; the wall does not have to be planed flat. What matters is that the board is stable and the tile can stick to it. Putting the same finish level behind tile that you put behind paint is throwing money away.
A bathroom is full of angles. Internal corners where two walls meet, external corners at the room perimeter, the line where ceiling meets wall — every one of these is a place where board meets board, tape meets corner bead, and the work shows if it is done carelessly. An internal corner in a bathroom, where humidity rises every time someone showers, has to be taped in a way that sits tight and does not work loose over years. An external corner — the arris edge that sticks out into the room — has to be straight enough to not catch the eye and strong enough to not dent.
Sanding is the part most people do not think about until they are living in a house with drywall dust in every room. New board is sanded to prepare for finish; joints are sanded after the compound goes on; edges are sanded after primer. If the house is occupied, every bit of that dust goes everywhere. Closed doors do not stop it. The dust gets into every surface, every light fixture, every piece of equipment. The contractor who cares about this part has a plan for containment before the first board goes up. The state a wall is handed over in — swabbed down and ready for paint, or still smudged with compound dust — makes the difference to how long cleanup takes afterward.
A wall that is out of plane — meaning the surface is not flat, but bows or ripples — shows in two completely different ways depending on what finishes it. Under paint, especially paint with any sheen and light raking across the surface, an out-of-plane wall reads as wavy. You see it every day. Under large-format tile, an out-of-plane wall creates tiling problems: the tile does not sit flat, or the grout joints widen where the wall dips, and the surface reads as sloppy. Small tile hides it better because each tile is small enough to conform slightly to the irregularity. This is why wall preparation — getting it reasonably flat before anything goes on top — matters more than most people realize.