No sooner had the "American Idol" and Grammy Award winner opened her mouth to sing "I Can't Give You Anything but Love" than she was gently stopped, mid-phrase.
The man asking her to stop? Nine-time Grammy winner Wynton Marsalis.
"That was very good," he told her. "I just want you to concentrate and do it again. Let your instincts take over."
They were preparing for "After Midnight," a musical celebrating Duke Ellington's years at the famous Cotton Club nightclub. Fantasia will be the star; Marsalis the musical director.
On this day - their initial meeting for the show around a piano in a studio at the home of Jazz at Lincoln Center - their roles were different. He was the professor and she was the eager student.
He taught her that Billie Holiday often sang using quarter note triplets, that you can usually detect African rhythms beneath most blues songs, and that to scat correctly would take at least two years of study.
"You need to check out Dinah Washington," the trumpeter and composer told his 29-year-old charge, who was dressed in black leggings and a T-shirt reading "Love Is the New Red." Marsalis wore a sharp gray suit with a pocket square.
Washington, he said, had an empathetic voice that seemed to always tell the truth. "You have that same quality," he told her. "When you listen to her, you'll notice that."
"Do you have any records that I could listen to?" she asked.
"Yeah, we'll give you a whole list," he said.
At one point, Marsalis nudged aside the rehearsal pianist at the Steinway to offer Fantasia his own half-dozen interpretations of the opening lines of "I Can't Give You Anything but Love," elongating some syllables, breaking up others.
"You see what I mean about varying your phrasing?" he asked.
Asked after the session how it was to learn so much, the singer gushed: "Excuse me when I say this, but it almost felt like a musical orgasm. I was just sitting there going, 'Oh my God!'"
An hour or so into the rehearsal, the duo moved on to their second song - "Stormy Weather," made famous by Holiday and Lena Horne. "This is a very different kind of song," Marsalis warned. "This is more a dramatic song."
Fantasia, who won 2004's "American Idol" and has put out four CDs including the new "Side Effects of You," let her voice get breathy as she negotiated the tricky standard.
"Rain pourin' down, blinding every hope I had," she sang. "This pitter and patter and beating, spattering drives me mad /Love, love, love, love ..." She trailed off.
"I lost the rhythm," she said, sadly.
"But I love the way you said 'love.' You made me believe something about love," said her teacher, immediately sending a wide smile blooming on Fantasia's face.
At one point during the session, the student seems to want to get something off her chest. "Can I tell you something about me?" she asked, embarrassed. "I don't read music."
Marsalis smiled. Did she think this scion of a family of world famous jazz musicians and a decorated musicologist with so many Grammys and a Pulitzer Prize didn't notice?
"I know you don't. You're not looking at the music when you're singing," he said, laughing.
But Marsalis doesn't judge - nobody read music in New Orleans when he was being raised. "It doesn't matter to me. I grew up with great musicians who couldn't read," he said. "My daddy always said, 'If you play music you don't need to read it.'"
Marsalis may have an encyclopedic knowledge of music but he's not above mocking himself. He tells the story of trying to impress Sarah Vaughan as a 22-year-old by playing the obscure Duke Ellington song "Tonight I Shall Sleep (With a Smile on My Face)."
When he was finished, the legendary Vaughan told the young man: "That's beautiful, baby, but it's in the wrong key. Let me show you how it goes."
At the session, Marsalis and Fantasia work on their approaches - and it is agonizing slow work. When the musical opens its doors at the Brooks Atkinson Theatre on Oct. 18, there will be 17 musicians in the pit and Fantasia and co-star Dule Hill and two dozen performers will be asked to capture the sound and glamour of the Harlem Renaissance.
But on this day, a single lyric gets the same treatment as a Biblical passage in Sunday school. Marsalis stops at one line - "If he stays away, old rockin' chair will get me" - and the two discuss its meaning. "Look into the lyrics of these songs. Think about it. There's more than meets the eye," he said.
Two hours after it began, the initial session in mid-August ends and Fantasia leaves with homework: lists of recordings and several vocal exercises. Student and teacher hug at the end. They'll meet again and again as she masters the material.
"I really want to make him proud," she said as he left.
This isn't the first time Marsalis and Fantasia have crossed paths. When she was 22, she joined Marsalis and the Jazz at Lincoln Center Orchestra as a guest vocalist for their 2007 fall gala. He recalled her as a woman with poise and skill.
"I remember. She was gracious and so young," he said after the session.
Of her current challenge, he's optimistic: "She'll be ready. She can already do so much on a very high level. She's survived in a very tough business. So with a person on her level, you just need to point them in the right direction. She'll figure it out."