Depending on the day, hour, and minute, Michelle Johnson is either living the dream or enduring a nightmare.

The velvet-voiced soprano, 29, is the latest slated-for-stardom singer to come out of Philadelphia's Academy of Vocal Arts, and was anticipating a relatively light spring schedule to prepare for her first Aida at the Glimmerglass Opera this summer. Then the Opera Company of Philadelphia suddenly needed a replacement soprano for the title role of Puccini's Manon Lescaut. When Johnson opens at the Academy of Music on Friday, she'll be performing an opera she began learning on March 26, less than a month ago.

"I did a couple prayers," she said, somewhat wearily, last week. "I'm still praying. I'm all prayed up."

Praying while playing Manon Lescaut is an exercise in irony: The character is a moral renegade, an 18th-century Frenchwoman who escapes from the convent to lead a picaresque life of intermittent wealth and glamour, only to die, exiled, in the badlands of Louisiana. Johnson grew up in Pearland, Texas, daughter of a Baptist minister. Praying for fallen women may not be all that foreign to her.

In strictly vocal terms, the singing is heavier than the Verdi roles she's used to. The soprano who was originally scheduled, Ermonela Jaho, had successfully navigated Puccini's Madama Butterfly a few seasons ago in Philadelphia but was harder to imagine as Manon Lescaut. The opera was Puccini's first big one — perhaps written at a time when the composer didn't know so much about pacing a major operatic role.

"Opera houses are very scared of this opera," said Opera Company music director Corrado Rovaris. "It's hard to find singers who, by the end of the opera, are still in good shape. … Sometimes they reach the last act completely done."

Johnson's past performances — Verdi's Oberto, Puccini's Suor Angelica, and any number of aria evenings presented by AVA — suggest that she has the vocal goods. Then there's the imprimatur of winning the Metropolitan Opera National Council Auditions in 2011. That's one reason OCP was willing to take a chance on Johnson — and why her teacher, veteran bass-baritone William Stone, has every confidence in her. He had a similar experience back in the 1970s with the premiere of Krzysztof Penderecki's Paradise Lost, when the composer was behind schedule and left him mere days to learn his role.

But the audience might have been less likely to hear mistakes in Penderecki than it will be in Manon Lescaut. So when Johnson called Stone from Houston, where she was wrapping up an Il Trovatore, he paused before offering advice.

"I thought for a second or two or ten. I was weighing in my mind: Is this a part that she can sing? Yes. Does she have enough time? Yes. Is she going to be in a supportive environment? A big yes. Everybody is buoying her up," he said.

"I said to Michelle, 'Singing careers are made on events like this. If you think you can do it, I certainly think you can do it.' "

"I could've made so many excuses," said Johnson. "But that's not me." Ultimately, her decision was based on faith: The opportunity wouldn't have fallen into her lap if she wasn't able to handle it. That faith seems to have spread to her colleagues. Stage director Michael Cavanaugh knew nothing about Johnson. And now? "I can't say enough wonderful things about her. She's the real thing."

And she's a talent that was wholly unanticipated. Now in her final year at the Academy of Vocal Arts, Johnson grew up in Texas, seized by opera while watching Madama Butterfly on PBS but knowing little about the art form. She arrived at the New England Conservatory with only a fraction of the repertoire her classmates knew, and convinced she was a mezzo-soprano. Now, after four years in AVA's boot camp, she's hardly an overnight wonder — and, luckily, is hardly less seasoned than her Manon Lescaut colleagues.

All the cast members are, like Rovaris and Cavanaugh, new to the opera, which is unusual in a major Puccini production. There's anxiety in that, but also relief: Even the most secure Manon worries whether she'll measure up to the last one her costar sang with, but "we don't have to look in the rearview mirror," says Cavanaugh.

Still, you won't hear Johnson rhapsodizing about how cool it is to learn the music and the staging at the same time. That's not how Diva OCD works: You get music in hand first, then determine the physical presentation. At times during rehearsals, she would sing a phrase with the authority of a veteran, then forget the words and hum her way through the rest. Her constant companion was a small notebook, with the words written in her own longhand. She could have used a photocopy of the libretto, but writing the words aids memorization.

By Easter weekend, she had hit overload and canceled vocal-coaching sessions to spend time outside with her dog, Jasper.

Both Stone and Rovaris knew the key to Johnson's success was that she not be overwhelmed. Stone had her count the pages in the score and divide the total by the time she had to memorize them. It came out to seven pages a day. "That's doable," he said. And Rovaris cautioned her against learning the role the way one crams for a final exam: That kind of knowledge might not be there when needed.

Theatrically, the role is tricky. Manon can come off like a none-too-lovable material girl. But Cavanaugh knows how to keep Johnson adorable: Her character takes girlish delight in the wonderful world around her.

"You see a shiny toy and you grab it," he says. "It's really hard to hate a child. You can't fault somebody for taking everything that's given to them. We've all seen people who, no matter how much they have, they always want more or something else. She has this push and pull between surface things and a profound depth of emotion."

Says Johnson, "She has girl power — in her own way. I'm a lyric soprano, so of course I'm going to cry and die at the end. But before that, I get to have fun."

And she isn't the type to be intimidated on opening night. Anyone who has seen her onstage can't help but observe how she morphs from an attractive offstage woman into an imposing onstage presence who seems to dare audiences to resist her. Time and again in the competitions she has entered, she was dubbed the audience favorite.

The fear among opera lovers is that Johnson's Stradivarius pipes might be damaged by too much, too soon. Given her vocal resemblance to the great Leontyne Price, one hopes that she'll have a 30-year career.

"So do I!" she exclaimed. "I thank God that I'm able to learn music very fast and that I can pull this together. But I want to protect myself. I really want to protect myself."

Contact music critic David Patrick Stearns at dstearns@phillynews.com.

Opera Company of Philadelphia Manon Lescaut 8 p.m. Friday and April 27; 2:30 Sunday and April 29; 7:30 p.m. Wednesday at the Academy of Music, Broad and Locust Streets. Tickets: $10-$225. 215-893-1018 or www.operaphila.org