Mezzo-soprano Mildred Miller has made her most memorable appearances at the Metropolitan playing boys' roles. In fact, one year all but one of her roles were that of a boy. That year, she said the other day, she returned home after appearing in a boys' role and destroyed all of her slacks in angry frustration. "It's so much more feminine to play the part of a woman". said Miss Miller. Her most complicated role has been that of Octavian in "Der Rosenkavalier". In most of the opera, she plays the part of a boy, but in one act she plays a boy dressed up as a girl trying to look like a boy.