Once the rehearsal process for a production at the Metropolitan Opera moves from basement rooms to the main auditorium, Donald Palumbo, the company’s chorus master since 2007, gets uncomfortable sitting down. He walks restlessly up and down the aisles, score in hand, watching the stage, conferring with the conductor, making notes in his head.

If there’s a break in the action, he races backstage to offer tips. That is how he tripped during a 2009 run-through of Rossini’s “Barbiere di Siviglia,” breaking a wrist. He conducted the offstage choruses in Verdi’s “Aida” later that day.

In a lull during a recent rehearsal for a rare revival of Shostakovich’s scorching “Lady Macbeth of Mtsensk,” which opens Monday, Mr. Palumbo, 66, came upon a group of choristers waiting for a cue. Amid the raucous music of the opera’s wedding scene, he had detected a slight imperfection. “Make sure that F-sharp in the tenors isn’t too high,” he announced.

It is tiny observations like that, arriving by the dozen, that have transformed the Met’s chorus over the past seven years, from an inconsistent ensemble in the shadow of the company’s world-class orchestra to an equal partner currently excelling in the most challenging fall season in years. “What’s palpably different is that the commitment and the vitality of the group have been ignited again, and it functions very, very thrillingly,” James Levine, the Met’s music director, said in a phone interview.