AIX-EN-PROVENCE, France — Barbara Hannigan was lying on her back onstage a few weeks ago during a performance of Debussy’s “Pelléas et Mélisande,” a dreamlike, demanding production at the Aix Festival here, in which Ms. Hannigan, playing Mélisande, barely left the audience’s sight.

Ms. Hannigan’s lithe body was still at first. Then, in eerily smooth slow motion, she began to contort her limbs, mashing the side of her face into the floor, arching her back and twisting her torso until she rotated and raised herself to standing — in four-inch heels — without propping herself on her hands or arms. It was an excruciating, mesmerizing spectacle, though hardly unusual coming from a singer as notable for her intense presence and choreographic control as for her voice.

Over the past two decades, Ms. Hannigan, 45, has wielded her pristine soprano, with its easy extension to the stratosphere, as an expressive tool in blazing performances, whether as Berg’s Lulu or in contemporary masterpieces like George Benjamin’s opera “Written on Skin” and Hans Abrahamsen’s wintry song cycle “let me tell you.”