In fact, barring the need for surgical removal of an appendix, the hands and arms can be an even bigger problem, since they are inescapably visible. Aspiring vocalists may take years to learn to use their hands expressively or, alternatively, to keep them out of mind, if not out of sight.
This sort of career realignment is rare among singers. Ms. Hannigan cites the parallel example of the French contralto Nathalie Stutzmann, who conducts and often sings at the same time.
The tenor and baritone Plácido Domingo is the best-known example of a classical vocalist who conducts regularly, though typically in the opera pit, where he is not singing at the same time.
But why not do both on the concert stage? Ms. Hannigan sees no reason not to, arguing that singers actually have an advantage over instrumentalist-conductors who play along with an orchestra: precisely that availability of hands and arms, otherwise idle, to lead the band.
“You sing like a conductor,” colleagues used to tell Ms. Hannigan of her musical personality and stage manner, she said in an interview on Sunday. And you can see some of that when she conducts while singing.
In three splendidly delivered Mozart arias here, her beating of time and cues to players often seemed mere amplifications of expressive gestures she might have made anyway. Since much (most?) of a conductor’s job takes place in rehearsal, she was able to leave a lot of the last-minute coordination and balancing of parts to the concertmaster.
Colleagues have also told Ms. Hannigan that her arm movements are particularly expressive, she said, and this, too, was borne out in her conducting of purely instrumental works: Rossini’s “Scala di Seta” Overture, Ligeti’s “Concerto Romanesc” and Fauré’s “Pelléas et Mélisande.” Her arms seemed fluid, elbowless entities, shoulder to wrist, evoking curvilinear waves of sound, and she did not use a baton.
This is one reason that Ms. Hannigan, unlike many other female conductors, chooses not to shroud her arms, or her femininity as a whole, in, say, a dark suit. She wore one for her conducting debut, at the Théâtre du Châtelet in Paris in 2010, and that was the last time.
“I thought, ‘I never wear a suit to sing,’ ” she said. “ ‘Never, ever. That isn’t my uniform.’ ”
She decided that from then on, she would wear “something that suited the music, suited the program,” she said.
For her concert on Saturday, she wore a glittery knee-length sheath until the grand finale, Ligeti’s “Mysteries of the Macabre,” a series of high-flying arias drawn from the composer’s nonsensical opera “Le Grand Macabre.” Ms. Hannigan, a specialist in modern and contemporary music who recently won great acclaim in productions of Berg’s “Lulu” and George Benjamin’s “Written on Skin,” has made this work — which she has sung often — her calling card.
She says she now sings it only when conducted by the English maestro Simon Rattle, a frequent and favorite collaborator, or — more often — herself. She performs it in vampy black leather, high-heeled platform boots and a wig, and inevitably brings the house down, as she did on Saturday.
Mr. Rattle has mentored Ms. Hannigan and sent her to the wizardly Finnish maestro and pedagogue Jorma Panula, who, Mr. Rattle says, came out of retirement to guide her.
“We’ve only worked together for eight or nine years,” said Mr. Rattle, who is in town to work with students of the Lucerne Festival Academy, “but it quickly became apparent that she is one of the best musicians out there. When she brought up the notion of conducting, I was surprised but not staggered. It seemed a fairly normal thing to do.”
A latecomer to conducting, Ms. Hannigan now faces the challenge of building up an orchestral repertory while maintaining a busy singing career in Europe and expanding it in North America. But Ms. Hannigan, with musicality to burn, says she is in it for the long haul.
“Conducting is now 20 percent of my schedule,” she estimated. “Eventually it will be 50-50, and then I will only conduct.”
She looks to tackle bigger orchestral works, with Mahler’s Fourth Symphony in the offing. (Ms. Hannigan will sing the soprano solo in the finale of that work in a performance here on Sept. 6, with Matthias Pintscher conducting the Lucerne Festival Academy Orchestra.)
She also looks to conduct opera, with Stravinsky’s “The Rake’s Progress” being a possible starting point.
For her and audiences alike, it promises to be quite a ride.