The joints between sheets, taped and built up in coats until the wall reads as one continuous surface.
Taping is the step that turns individual sheets of drywall into what reads as one continuous wall, and it is also the step most likely to be rushed. Paper or mesh tape goes into the first coat of mud over every joint and every screw head, then subsequent coats widen out from there — each one covering more area than the last, feathered thin enough at the edge that there is no ridge to sand out later. Two coats might look fine the day the painter shows up. Three, done properly and sanded between each, is what holds up once the wall has natural light crossing it at a low angle.
Inside corners, outside corners with metal or vinyl bead, and the transition where a ceiling meets a wall all behave differently under mud and need to be worked differently. Corner bead that was not fastened tight before mudding will show a hairline gap forever. A joint mudded in one thick pass instead of several thin ones tends to crack as it dries, because thick mud shrinks unevenly. None of this is visible until the paint goes on, which is exactly why it is worth getting right the first time rather than finding out after.
Describe the room, the texture, and whether water is involved, and we will match you with the right contractor.
Get Matched With a Drywall ContractorA rough description is plenty — we will follow up on texture, size and finish level.