“I certainly wouldn’t want to be taking on Isolde,” Deborah Voigt told an interviewer in Opera Monthly in 1992. Reminded of her statement, Ms. Voigt laughed merrily in an interview last week. “My, how things change!”

Tonight she does just that, making her Metropolitan Opera debut as Wagner’s torturously sublimated and then cataclysmically released romantic heroine. This is only Ms. Voigt’s second time singing the role. She made her debut in the part five years ago at the Vienna State Opera. Now 47, Ms. Voigt has built her career by singing a very wide variety of roles in the Italian and German repertory, but she has concentrated on what are considered the lighter Wagnerian heroines — Elsa in “Lohengrin,” Sieglinde in “Die Walküre,” Elisabeth in “Tannhäuser.” Only now she is venturing fully into the most taxing Wagnerian territory, in which Isolde, an Irish princess of medieval legend, qualifies both musically and dramatically.

The catalyst for the artistic change had its roots in Ms. Voigt’s physical transformation. In 2004, Ms. Voigt had a gastric bypass operation that enabled her to shed 135 pounds. “I think you couldn’t see as much of what was going on with my face because there was just too much,” she said of her appearance before the operation. “And the same thing with gestures. And, of course, then costuming gets to be a problem because they think, ‘Well, let’s just throw a tarp on her,’ and then you really can’t see what’s going on. So I’m finding it’s a lot of fun now not to have to worry about that.”

Roles that she would have performed previously only in concert she has now tried in full-dress production. Last year at the Met she finally sang Helen of Troy in Richard Strauss’s “Die Aegyptische Helena.” In 2006 she sang Strauss’s “Salome” at the Lyric Opera of Chicago. To prepare for her “Dance of the Seven Veils” she worked with a gym trainer and was coached in the lexicon of Arabian veil dances by choreographer Jane Comfort. Now, for her Met Isolde, Ms. Voigt insisted on new costumes that would make her svelte shape visible.

But first and foremost is always the music. Talking in her Met dressing room after a rehearsal late last month, Ms. Voigt said that it is, perhaps above all, the length of the heroic Wagnerian roles that makes them so daunting, so taxing, so treacherous to try too early in a career. “It takes a long time for singers to learn how to pace themselves,” she said. Yet Wagner supplies dynamic peaks and valleys that make it possible. “We have our idea in our heads that it’s a huge orchestra and you have to scream, scream, scream, scream, scream. And that’s really not the case. There are many moments of lyricism.”

“I don’t have a Birgit Nilsson sort of voice,” she said, referring to the great Swedish soprano, whose voice seemed to slice through Wagnerian orchestrations, “so I’m very much trying to find those places that don’t require so much stentorian kind of sound.” The passion of Isolde’s role, however, can make it difficult to hold back. Rehearsing the first act the day before, she found herself engulfed in her heroine’s storms and rages to the point of no return. “I had to say, ‘Wait a minute, Voigt, you’ve got five hours ahead of you. Easy does it.'”

Isolde is one of the most complex heroines on the operatic stage. Her lover Morolt has been murdered in combat by the Round Table knight Tristan, but she finds herself unable to avenge his death when given the chance. Instead, her medicinal arts bring Tristan back from the point of death, but her reward is to be sent back to Cornwall as a marital prize for Tristan’s aged uncle King Marke. Aboard a ship in Act 1, Isolde shames Tristan into sharing a death potion with her by way of reparation. Instead, her handmaiden Brangane substitutes a love potion, which frees the ardor that Tristan and Isolde’s mutual antagonism previously suppressed.

As far as physical acting goes, Isolde is no verismo heroine like Puccini’s Tosca, which Ms. Voigt sang at the Met in 2006. Isolde’s role instead is dominated by brooding, seething introspection, and she is a symbolic as much as psychological incarnation. Ms. Voigt spoke of her admiration for the way that the Met’s “Tristan,” directed by Dieter Dorn, supplies gestural illustration to the mystical elements of Isolde’s persona. Ms. Voigt’s Tristan is Ben Heppner, who sang the role here at the Dorn production premiere in 1999. “Ben is so great,” Ms. Voigt said. “He’s done the part so many times, and he knows this production very well, and he’s really fun to play off of.” As she prepares for her second go-around with Isolde, she finds herself more sensitive to the dialogue between her and Tristan — “the undercurrent of what’s being said and what’s really meant” — which is dichotomous throughout Act 1.

Ms. Voigt supplemented her Met rehearsals by studying the live-transmission DVD of Jane Eaglen and Mr. Heppner in performance at the Met in 1999. She also listened to the recorded Isoldes of Nilsson, Kirsten Flagstad, and Margaret Price. “Then I kind of put them away,” she said. Ms. Voigt is wary of being influenced by what other interpreters have done. “I’m quite a mimic, and I pick up vocal mannerisms very easily. I find it much more interesting to sit with the score myself and play it through my head.” At home after the day’s rehearsals, she will often work with the score at her piano, or simply read through it, sometimes to a background-noise obbligato supplied by her television.

In this Everest of stamina and intensity, Isolde’s highest peak remains her final one, when she brings the opera to its conclusion with the five-minute “Liebestod.” Having begun the opera poisoned with vengeance and self-delusion, she now surrenders to death while Wagner’s orchestra surges rapturously. But Isolde is offstage for most of Act 3, and the “Liebestod” thus comes after a long period of silence. “That’s intimidating,” Ms. Voigt admitted. During waits like these she enjoys having a few laughs outside her dressing room with the Met’s dressers. To ensure that her voice remains pristine, however, she will go back to her room about 20 minutes before her Act 3 entrance. She’ll do some final vocalizing. And then she’ll return to the stage to join the fatally-wounded Tristan in eternal union.