A dozen multigenerational singing actors team up for a mercurial two-act collage of Sondheimiana, ranging from choice outtakes from Follies to “A Weekend in the Country,” the most lavish and Mozartean of the composer-lyricist’s great ensembles. Familiar faces from Broadway include Emily Skinner, who dips “The Ladies Who Lunch” in liquid nitrogen, and Conrad Ricamora, translucent in “Finishing the Hat.” Prime among the Washingtonians you’ll be glad to meet are Donna Migliccio, withering in “Could I Leave You,” and Awa Sal Secka, luscious in the seldom heard “All for You.” Though cast as a supporting player, the budding matinee idol Nicholas McDonagh breaks out time and again on the strength of his musicality and roguish ease. In tandem with Bobby Smith, a mellow character tenor with a few miles on him, he lights up the stage in the seldom anthologized “Impossible,” a vaudevillean sparkler from A Funny Thing Happened on the Way to the Forum. —M.G.