A corner defined where a wall panel and a ceiling panel meet is trimmed by a crown molding assembly comprising a wall platform, a ceiling platform, a crown molding member, a wall edge-trimming bead, and a ceiling edge-trimming bead. The wall platform is affixed to the wall panel, at or near the corner, and has an expansive surface spaced from the wall panel. The ceiling platform is affixed to the ceiling panel, at or near the corner, and has an expansive surface spaced from the ceiling panel. The crown molding member is affixed so as to bridge the expansive surfaces of the wall and ceiling platforms. The wall edge-trimming bead is affixed so as to cover an outer edge of expansive surface of the wall platform. The ceiling edge-trimming bead is affixed so as to cover an outer edge of expansive surface of the ceiling platform. Each platform may comprise a drywall layer or stacked drywall layers. If either platform defines a step, the crown molding assembly further comprises another edge-trimming bead, which is affixed so as to cover an outer edge of the step.