The Met, the largest performing arts organization in the nation, has struggled with unsteady musical leadership for more than a decade. Years of health problems kept Mr. Nézet-Séguin’s predecessor, James Levine, away for long stretches, and ultimately forced him to step down as music director. Then accusations of sexual misconduct, which Mr. Levine has denied, led the Met to sever all ties with him earlier this year. The new music director has big shoes to fill, and a big wound to heal.

Mr. Nézet-Séguin, who had originally been set to assume the post in 2020, moved up his start date to take a stronger musical hand at the opera house after the allegations against Mr. Levine came to light. And although it will be a few seasons before he takes on his full workload at the Met and implements some of his plans for commissions and collaborations, he is already making his presence felt. At his first “Traviata” rehearsal with the orchestra, he paused often to fine-tune passages, even though the company has performed the piece more than a thousand times.

He has been helping the singers hone their roles — especially the star tenor Juan Diego Flórez, a bel canto specialist known for his high C’s, who is singing the somewhat lower and less elaborately ornamented role of Alfredo for the first time. Mr. Flórez, whose Alfredo promises nevertheless to be very much in the bel canto tradition, praised Mr. Nézet-Séguin’s understanding of voices. “He wants to bring back the attention to detail that this part has,” he said in an interview.

When Mr. Nézet-Séguin urged Mr. Flórez to sing the drinking song in the first act softly, as the score indicates — to underscore that Alfredo is still insecure and sexually inexperienced — he drove the point home in a characteristically good-humored way.

“Like a vir-ir-ir-ir-gin,” the Met’s new music director sang, channeling Madonna.

During rehearsals Mr. Nézet-Séguin worked closely with the creative team to align the staging with the music, tweaking the blocking in one duet to help the singers and making sure the motivations of the characters were rooted in the score.

“He’s a real dramaturg, musically,” Mr. Mayer said shortly before the start of the first stage rehearsal. “He’s very invested in, and interested in, the narrative information that is conveyed through music that is unrelated to the libretto.”