I want to share a great tip that I got from the Construction Dept.
It is really difficult to construct a set on the stage and have the finished walls end up absolutely plumb. Usually it goes unnoticed. In a set with crown mouldings and ceiling flats, unfortunately it is a common problem that suddenly becomes visible.
Because the ceiling flats are wild (unsecured, for camera) and because they undergo installs and uninstalls, what often happens is the movement exacerbates the imperfections, causing a void or uneven gap where the crown is supposed to meet the flat. It never fits exactly like it did before it was removed.
The trick to reduce the obvious, is to install the crown 1/4" below the ceiling flat, establishing a deliberate void all around. The flats are supported by the walls, not the crown. It works, because the eye picks up void/absence-of-void, as opposed to smaller-void/bigger-void.