When I recently posted that Patti Lupone will be starring on Broadway in Sweeney Todd, I neglected to unearth that the production is a John Doyle creation complete with his trademark of casting actor-musicians (dreamed up originally to save money on hiring an orchestra). Lupone, who played sousaphone in her high school marching band, will be hoisting a tuba onstage. Michael Cerveris in the title role will put down his straight razor (we hope) to sing along with his own guitar accompaniment. And the musicians’ and actors’ unions have agreed on a compensation plan for this overlap arrangement.