Not only careful communication is essential, but we would greatly recommend, when you pick out your type and brand of paint that you go and pay for it, with the contractor's discount, which is given by Sherwin-Williams and most paint companies. Then you will know how much paint you purchased and the coverage. If it is possible, inspect the work of the painter daily.
Freshly painted walls often look blotchy. The color is uniform, but the sheen isn’t consistent. This usually occurs over the holes and cracks you patched with a filler or drywall compound. The porous fillers absorb the paint, dulling the surface (a problem called “flashing”). When light hits these dull spots, they stick out like a sore thumb. The smooth patch also stands out in contrast to the slightly bumpy texture of the rest of the wall. A quick coat of primer is all it takes to eliminate ﬂashing and texture differences.
As a former professional painter, something that drives me crazy is when I see painters cut in around wall plates. First of all, the amount of time that it takes to cut in is usually much longer than the amount of time that it takes to simply remove the wall plate. Secondly it does not look nearly as clean as it does when the wall plate is removed and thirdly it can invariably leave paint on the wall plate itself which also looks sloppy. Make sure the painter takes them off and then reinstalls them. It's less time.