Fans who have grown accustomed to Ms. Graham’s sweetness, both onstage and off, might be caught off guard by “Regina”; it demands an vocal ugliness that would drive some singing teachers insane. And her soothing stage presence takes a turn for the venomous here.

“The character fits me scarily well,” she said. “I must have an inner Regina.”

If Ms. Graham is exhausted — and she is, sometimes sleeping for nearly 12 hours after rehearsals — she doesn’t show it. Local reviews have been laudatory, and her voice sounds as powerful and assured as ever. In her trademark way, she somehow manages to make the irredeemable Regina sympathetic, even tragic.

“Susan’s still in her prime after 30 years,” said Stephen Lord, the production’s conductor and the festival’s music director emeritus.

Casting this opera has always been a challenge — one that has deterred companies from staging it. “The only reason to do ‘Regina’ is if you have the people to do it,” Mr. Lord said. Opera Theater’s lineup is starry: James Morris, a Wagner veteran, is singing the role of Regina’s brother Ben; Ron Raines, a Tony nominee for the most recent Broadway revival of “Follies,” is her other brother, Oscar; and the Metropolitan Opera fixture Susanna Phillips plays Oscar’s wife, Birdie, the story’s tragic heart.