The decisive moment went something like this: Mezzo-soprano Susan Graham and conductor Yannick Nézet-Séguin - who became instant friends when she interviewed him on a Metropolitan Opera HD simulcast - hatched a plan for her to open the Philadelphia Orchestra's three-week Paris Festival this Thursday with Songs of the Auvergne.

Written in Occitan - mixed with Spanish and lacking standardized pronunciation - the folk songs collected and adapted by Joseph Canteloube are among the most beloved in the repertoire. That accounts for what music from southwestern France is doing in a festival dedicated to Paris.

Of the 30 playful, homespun songs, Graham will sing seven for the first time ever. She floated the idea of using a modern French translation. "But Yannick said, 'It's so much more interesting to sing in dialect,' and I have to agree," she said with a certain amount of resignation. "It's a new language to wrap my head around. . . . We don't take the easy way out."

It's yet another instance of a career that won't let go.

At 56, Graham is giving up the "trouser" roles that made her famous - one reason she's not in the Metropolitan Opera's forthcoming Der Rosenkavalier. The age difference in playing a teenage boy is one reason; the physical demands are another. "We mezzos have to endure a lot. We're always kneeling to sopranos, jumping out of windows. . . . In the last Rosenkavalier [at the Met] simulcast, I got a charley horse in the beginning when I hopped off the bed. It felt like an ice pick in my leg."

She's also trying to have a married life for a change, having wed a college boyfriend in September. But in the fall, while she was trying to set up house in Burbank, Calif., to cement her relationship with her two stepchildren, the Lyric Opera of Chicago production of Les Troyens needed rescuing after Sophie Koch canceled. Among major singers, Graham has the epic role of Dido to herself.

In contrast, Songs of Auvergne, assembled between 1923 and 1955 and containing love songs, lullabies, and nature studies, has many illustrious interpreters. Kiri Te Kanawa solidified her recording career with her lush, soft-focus recording in 1982. Graham's friend Frederica von Stade staked her claim to them eight years later. The acclaimed 2005 Veronique Gens recording has the advantage of firsthand knowledge of a part of France that Graham has seen only from TGV train windows.

Many Americans wouldn't know Auvergne even exists were it not for the Canteloube collection. Visitors en route are warned to bring a good book because so little seems to be happening in towns that still have their medieval walls, towns where the old stay and the young are moving away, presumably due to lack of economic opportunities.

The rocky, volcanic land seems to exist in the past tense, with ruined chateaus every 10 miles or so. No wonder so many songs are about humble shepherds. Or not so humble. One of the most popular songs is titled "Bailero" (sometimes spelled "Bai'lero"), a hard-to-translate word that means chief shepherd, but with an affectionate word ending that balances respect with loving familiarity.

It's been said that only French listeners can truly understand these songs. And yet, "Bailero" - with its aching lyricism and words about separation from a loved one by a wide river - has begun to be heard at American funerals. Notions of regionalism are being continually revised in an internet-connected world. Though Te Kanawa's recordings gained popularity more for their lush vocals than for linguistic authority, Graham's claim on French style and enunciation has been endorsed over the decades by Gallic musicians and audiences.

But language is only the foundation. Musical imagination, says Graham, is her personal responsibility. "Singers are always inventing their own relationship with the fish, flower, hunchback, or whatever is in the songs," she says. "You decide how you feel about it and what you want the audience to feel.

"There's this Bizet song ["Le Grillon"] about crickets at twilight. The piano is making cricket sounds, and you have to invent where you are, what time of the day it is, if the crickets scare you or if you're a fly that wants to eat them," she says. "It's fun to let the imagination run wild. I have to become the character. Is it a boy or a girl? Old or young? Right or wrong, you have to have a point of view."

Inhabiting the songs is particularly crucial with Songs of the Auvergne. If the performer seems in the least bit detached, the pieces will come off like quaint tourist music, as opposed to anything real. "They benefit from a real investment," she says.

Choice of vocal tone is also crucial. In fact, that is what has kept Graham away from Carmen, the most famous French opera role of all. By the time she developed a true Carmen sound, she was too old to play such a volatile gypsy temptress. With the Auvergne songs, there are two hugely different precedents: Te Kanawa, with her supersaturated tone quality, and, looking back to 1930, Madeleine Grey's slim, vernacular voice that's better able to capture the peasant quality of the songs. "I hope to fall somewhere in the middle," Graham says.

And she can afford to. Younger singers sometimes strive for a beautiful sound as insurance for success. But vocal airbrushing maybe isn't appropriate for Auvergnat peasants. Or for the new opera characters Graham is taking on.

Having premiered the Jake Heggie opera Dead Man Walking in the central role of Sister Helen Prejean, she's portraying the convicted killer's beleaguered mother (first sung by von Stade) in the forthcoming Washington National Opera production, Feb. 25 to March 11. She's happy to let the larger role go to Joyce DiDonato; it has painful personal significance. You'd never know that from her live recording, which contains some of her best-ever moments - including a moment when Sister Helen is praying.

"My father died the day that was recorded," she said. "I didn't know at the time. They kept it from me. But I knew he was in the hospital. I'd like to take credit for great acting, but I was living the part."

Graham will also reprise the anguished Countess Geschwitz in Lulu at the Metropolitan Opera in future seasons. Again, vocal beauty isn't appropriate. "Everything doesn't have to be in pear-shaped tones," she says. "I'm not afraid to make ugly sounds if that's what is called for."