Somewhat contrary to Ralph's admiring description of the fascinating 1907 Fledermaus the singers did have important operatic careers. Do look them up, particularly the lead sopranos and tenor! If anything, there is a lesson here about the easy versatility expected in German theatres, where many staples of the repertoire relied on ability to act. Some singers started in operetta or spöken theatre - it still happens if you are in a smaller theatre which combines genres.