But, unlike the early concerts in the series, it would be hard to argue that the acting and dancing play second (or eighth) fiddle. None of the lead actors read from the scripts on music stands; they are fully costumed, they and the ensemble are fully choreographed. Encores no longer sees its primary mission as offering a second listen to long-dormant musicals with great scores and poor books. One now suspects that the producers are always hoping for a Broadway transfer, such as happened, most spectacularly, with “Chicago.” Even officially, as its page on the City Center website puts it, “This series of concert stagings revisits the archives of American musical theater…spotlighting the vocal talents of star-studded ensembles.” It’s hard to complain when the vocal talent is so abundant, as in “Once Upon A Mattress,” and it’s matched by so much comic talent.