"The girl’s on time, right?" says Gwen Stefani to Blake Shelton. The 44-year-old No Doubt singer looks femme fatale-radiant in a black-and-white polka dot Saint Laurent blouse.

"Well, you’re not the biggest girl here," Blake Shelton, 38, dead-pans in his Oklahoma drawl, taking a trademark dig at fellow coach Adam Levine.

Shelton’s not just busting chops: He’s revealing something fundamental about The Voice, NBC’s workhorse singing competition, which Stefani and Pharrell Williams, 41, are joining for its seventh season. Making "big girls" (or big boys) of its coaches has become its very reason for being. After all, despite its initial raison d’etre, the show, three years in, has never launched an actual star along the lines of American Idol alums Kelly Clarkson or Carrie Underwood. But those days were nearly a decade ago, and the idea of small-town-kid-discovered-on-TV is a concept that actually seems positively quaint in an era in which Vine and YouTube routinely transform amateurs into music celebrities (just ask Shawn Mendes and Tori Kelly, who were discovered through these mediums). So the show’s greatest use (despite the pleasure in seeing unknowns succeed, or fail, in assorted renditions of cover tunes) has turned out to be something else: taking established artists and making them bigger — much bigger — while showcasing a celebrity chemistry using high-caliber names that usually don’t tread in prime-time reality.

Billboard Cover Story Preview: 'The Voice' Coaches Discuss Season Seven & More

Levine, 35, has admitted that he and his band Maroon 5 were slumping when he signed on to The Voice in 2011 for an initial three-year term. Now he has a clothing line with Kmart and two fragrances; Maroon 5’s first post-Voice studio release, Overexposed, debuted at No. 2 on the Billboard 200 and its latest, V, now rules the Sept. 20 chart with sales of 164,000 copies, according to Nielsen SoundScan. Shelton, who was little-known outside of country when he was brought on, benefited from a similar career super-sizing: His seventh record, Based on a True Story..., released after the season-four premiere, debuted at No. 3 on the Billboard 200 and sold 199,000 copies sold its first week. JCPenney is sponsoring his current tour.

For NBC, Shelton and Levine were a stroke of good casting, a steal from under the noses of American Idol as it reinvented itself (poorly, most would argue) post-Simon Cowell, as well as Cowell’s own The X Factor. Their characters exude the sort of authentic-feeling pop star noblesse oblige that has established The Voice’s tone — and signaled to the likes of season-six coaches Usher and Shakira, and now Stefani and Williams, that The Voice is a ready-made form of brand extension. "Shakira was big — in a way — before," says Paul Telegdy, NBC president of late-night and alternative programming. "She’s a household name in America now." As Levine himself says: "The power and impact that a show like this has for a band like [Maroon 5] is as potent as it was to be on The Ed Sullivan Show."

New mom Christina Aguilera, 33, is off this season. She was the show’s biggest "get" in its infancy, and rumored to have been paid the most among the coaches: $12 million (a figure several industry sources suggest is exaggerated). She pops in and out of the coaching chair, but, like the others, makes the most of her face time. Last November, the day after pop duo A Great Big World rereleased its track "Say Something" with Aguilera joining on vocals, they all performed it on The Voice. Sales of the song shot up by 1,761 percent by the end of the week.

'The Voice' Billboard Cover Shoot

Nonetheless, coach-boosting narrative aside, The Voice suffers from an obvious storytelling glitch. As a dream factory, it has anointed six winners — and they’ve all since vanished like loved ones on The Leftovers. "Who does better? The Voice judges or the Voice contestants?’ " carped Cowell to The New York Times in May. “It’s quite obvious the judges have sold a ton more records.” (In fact, a Voice coaching gig is almost certainly more coveted than a turn as an Idol judge.)

The coaches insist that the hopefuls get something residuals can’t buy: a dose of hard-earned wisdom from their mentors. In October 2012, Levine said, "Eventually, The Voice is going to have to launch somebody into the stratosphere to continue to be taken seriously." But now he argues that the show is not about making a star so much as helping budding singers become as "well-equipped as possible for reality, which starts the minute that confetti falls and people continue with their careers."

Williams interrupts to point out what an invaluable service their celebrity wattage provides: "I just don’t like that we’re talking about this from the defense, as if there’s something wrong. I never had a Gwen Stefani, or an Adam Levine, or a Blake Shelton to come in and tell me anything when I was 15 years old."

The truth is, The Voice does shower its riches on the coaches — and on NBC. The network finished No. 1 in adults 18-49 in 2013, but even with the hit James Spader drama The Blacklist and football, it remains deeply dependent on its singing competition show. (Between originals and repeats, The Voice powers more than 90 hours of the network’s prime-time schedule a year.) Pre-Voice, NBC was languishing in last place among the networks.

But lacking back-end syndication riches, all hit reality shows get overworked as executives weigh the risk of viewer burnout. The Voice’s audience, particularly in its last two spring cycles, has declined from season-long averages of more than 14 million viewers, according to NBC. The drop-off has occurred as other cover-song shows, including Cowell’s since-canceled The X Factor, ABC’s Rising Star and Fox’s Idol, have experienced varying degrees of catastrophic ratings performances.

'The Voice' Season 7 Trailer Features Gwen Stefani, Pharrell Lovefest

When it debuts on Sept. 22, The Voice will see whether America still has an appetite for the genre. Star-wise, the show is potentially beginning season seven at a deficit, with Levine’s mock kid sister gone missing. (Aguilera also took time off in 2012 and 2013.) To freshen the brand, Stefani and Williams were chosen to replace Usher and Shakira — though "replaced" is not a word anyone around the show uses. Shakira may, supposedly, return, after leaving to have a baby (she’s now pregnant with her second child); Usher is promoting his upcoming movie, Hands of Stone, in which he plays the boxer Sugar Ray Leonard, and readying a new album and tour.

"If you remember [the 2011] season after Simon Cowell left American Idol, it was game over," says Telegdy. "We decided we would have a strategy around evolution and change."

"Evolution" and "change" are not words that TV executives normally like to use to describe their franchises; a new way -forward can also be a risk to everything that has been built. The hope is that Stefani will be a triple-threat female-viewer magnet — pop star, fashionista, mother of three. (This writer is reminded multiple times on-set that she is breast-feeding; she gave birth to her third son, Apollo, in March). After Shakira, a wildly popular Latin artist, Stefani will need to bring her own fan base: presumably, moms old enough to recall No Doubt’s first hit single, "Don’t Speak," in 1995, and, perhaps more importantly, the tween girls who wear Stefani’s fashion lines.

Williams may have a global hit in "Happy," but he’s the season’s most likely weak link. NBC says it has been after him, with his artist-producer gravitas, to do The Voice for a while. But within the show, says a person familiar with the strategy in choosing coaches, executives were concerned whether the Grammy-winning producer, who has worked in-studio with some of pop’s biggest names (Justin Timberlake, Daft Punk, Alicia Keys) would seem too serious, projecting an aloof brand of cool that wouldn’t play alongside the hammy Shelton and Levine.

Several months into taping, says Audrey Morrissey, one of the show’s executive producers, Williams assuaged doubts that he wouldn’t submit himself to the theater of the game. She claims he has become a competitive foil to Levine, who in previous seasons, using his “Sexiest Man Alive” charm offensive, could bank on convincing most of the promising young female pop singers to chose him as coach. "He is insanely competitive," insists Morrissey of Williams. "And he will go after something. For the first time, Adam has a real run for his money in his lane of music."

Busy celebrities that they are, prospective coaches don’t actually do test shows to see how they would jell with returning cast members. And choosing coaches is not as simple as brainstorming the biggest names out there and approaching each with an offer. For starters, the show involves committing to filming some 40 days over six months. And for some, the idea of doing a network TV game show is simply a non-starter. "Cool is as cool does," is what Williams says to the perception that he’s selling out by doing network TV. "I do things because I really want to do them. Listen, what you eat doesn’t make me shit." (Though Stefani met with two key show executives before agreeing to join the series, Williams’ deal was negotiated from afar. It helped that he had been a mentor on Team Usher previously.)

Still, NBC seems unsure how to package Williams. In an expensively produced Old West-style ad to promote the new season, Shelton and Levine are gunslingers, and Stefani is the bad-ass biker chick. Williams, in a poncho, is a short, mysterious figure with soft features. He won’t be returning for the spring, so perhaps the enigmatic loner role is apt. "I consider this another great musical endeavor," he says. "I’m still in the studio with Ed Sheeran. I’m still going in with Gwen. This is a studio session to me."

Stefani sees somewhat of a higher purpose to doing the show. "In my life and my career," she says, "yes, I could just go back in and try to make another record right now with No Doubt, and see what happens while I’m in between breast-feedings and up all night. But this really shakes it up for me. This is inspiring in a whole new way."

The Voice, of course, once had another established coach: Cee Lo Green. In 2012, he took a hiatus after being accused of giving ecstasy to a dinner companion who said she later woke up in his hotel room. Green pleaded no contest in August to felony charges stemming from that incident — then tweeted, to outrage, "People who have really been raped REMEMBER!!! So if I TRIED but did NOT succeed but the person said I DID then what really happened?" "He remains a friend of the company," NBC’s Telegdy said before the tweets appeared, when asked if Green might ever return as coach. "For The Voice right now, our journey comes to an end.”

Mark Burnett, the producer who developed The Voice, likes to emphasize that it’s a family show: Leave the music business shenanigans to the likes of Mariah Carey and Nicki Minaj, whose disastrous turn as American Idol judges included a Twitter war.

With salary numbers like Aguilera’s rumored $12 million free for anyone to covet, Billboard asks Shelton and Levine if they renegotiate their deals with the show from cycle to cycle, as both Telegdy and Burnett had suggested they do. "I like my job," says Levine. "I don’t think I’m going to answer that." Later, Shelton, noting that he turned down The Voice twice before finally agreeing to sign on, says, "I would never take a cycle off. I think I would just quit. I don’t step away from country music and then come back to it, and I don’t think I would do that with this gig either."

However much Shelton is making, it’s enough that a rumor recently arose that he had turned down $1 million for him and his country star wife, Miranda Lambert, to play a single weekend at Caesars Palace in Las Vegas. (Shelton, through a representative, denied ever getting such an offer.) Lambert herself found fame as a contestant on TV’s Nashville Star, a fact in which Shelton takes a lot of pride: "To me, Miranda is the most important country artist to come out in the last 20 years. And she came from one of these [music competition] shows. That’s how she got her record deal. So anybody who calls out people for being on these shows can kiss [the collective] ass of some of the most important artists that we’ve seen in all of music, as far as I’m concerned."

Shelton has the kind of nice, easy buttermilk drawl that makes you want to believe everything he says. And yet his comment can hardly be taken as a defense of his show, the last music competition you’d expect to discover the next Miranda Lambert. On The Voice this season, it’s Stefani and Williams facing the biggest blind auditions of their lives. And for the next superstar, maybe they’re better off trying to find his or her video on YouTube. But millions of people will still be entertained and watching.