“The black Gypsy girl in my ‘Diary of One Who Disappeared’ — that was you,” Janacek wrote in one of his more than 700 letters to Stosslova. “That’s why there’s so much emotional fire in the work. So much fire that if we both caught on, we’d be turned into ashes.”

Mr. van Hove said: “I always adore when an author writes something that is a matter of life and death, which this is, I think, to Janacek. Some of my work, like ‘Electre/Oreste’ or ‘Boris Goudonov,’ is very political. But this one is about why we are here on earth, what is this life of ours?”

Pulling out a few sheets of paper covered in neatly written notes, Mr. van Hove said that he always began a project by writing down his initial thoughts. “It’s important to remember why you want to do it,” he said. “This piece is really human. It’s not about gods and dramatic events; it’s about things that everyone has experienced. Everyone has been in love with someone who hasn’t been in love with him or her, or has experienced platonic love.”

He added that he had long been a fan of Janacek’s music; his Flemish Opera production of “The Makropulos Case” in 2002 was one of his first forays into opera. He suggested “Diary” to the Muziektheater Transparant, a small, innovative opera company based in Antwerp, Belgium, with which he has long collaborated; he wanted to bring out Janacek’s ability “to write dialogue that became music, to turn language into notes. Even when they are singing, they are talking.”

He and Mr. Versweyveld began to read Janacek’s diaries, and his letters to Stosslova. (The relationship remained almost entirely platonic, and most of her letters were destroyed by Janacek, at her request.)