NEW YORK — Swaying precariously from side to side, sinking to her knees and finally collapsing altogether, Elza van den Heever puts on a virtuoso physical display to match her vocal fireworks as she stops the show near the end of Mozart's "Idomeneo."

It's the final aria for the character of Elettra, who has just seen her hopes of marriage to the king's son dashed. She's in a rage that can aptly be described as "operatic," and van den Heever savors every minute of it.

"It all comes very naturally," the South African-born soprano said in an interview in her dressing room before a performance last week at the Metropolitan Opera. "It's just this great gift Mozart gave to a soprano who's wanting to be physically active onstage."

Van den Heever said she worked out the elaborate movements for the aria with stage director David Kneuss, who oversaw this revival of the 1982 Jean-Pierre Ponnelle production.

"He gave me the rough sketch and let me fill in the rest," she said. "He said: 'I want you here on your knees, here you have to get up, here you bang on the altar, here you have to be on your knees so these guys can help you up, and here you have to faint and sort of have an epileptic attack at the end and you'll be carried off by six guys.'"

All this while singing an aria full of dramatic explosions and coloratura calisthenics.

Audiences worldwide can see and hear how van den Heever carries it off Saturday when "Idomeneo" is broadcast into movie theaters as part of the Met's Live in HD series.

More than many sopranos, van den Heever injects humor into her rendition of the aria, her over-the-top antics drawing laughter from the audience at several points.

"I felt that we would have the license to just go there," she said. "If I look at what Mozart wrote on the page, that going up to a C and then coming back down — to me it's like hysterical laughing. So it seems to me he wants you to go into this kind of out-of-body experience. So that, with David's liberties and giving me rein to do what I want to do, yeah, I think it's fun, I think it's not serious."

Van den Heever said she feels "a deep sense of sympathy" for her character, a Greek princess who has taken refuge in Crete, because "she's an outsider, one of those individuals who will never fit in."

"I know what that feels like," said van den Heever, whose powerful voice is matched to a striking, 6-foot-tall physique. "I was always a very awkward kid. I've been this length since I was basically 11. I always stuck out like a sore thumb, and with that comes a lot of ridicule."

In her career, she has tried to turn this to her advantage. "I would call myself a little socially awkward," she said. "But onstage I see that as a big plus. I'm able to just be somebody else and not worry about what people think of me. I give myself the freedom to just go there."