Meeting face-to-face after an intense, six-hour rehearsal at S.F. Opera for her role debut as Emilia Marty in Leoš Janáček’s The Makropulos Case, soprano Karita Mattila no doubt would have preferred to luxuriate in a bubble bath. Nonetheless, the great singer-actress, who recently showed her all onstage in Salome, slowly revealed herself. Speaking in the opera house, the diva shared thoughts on her imminent debut, ill-prepared conductors, and life at 50.