"Gossip Girl" star Leighton Meester is cooling her heels.

No, not the black patent heels she'll be perched high atop for her first scene of the day. That's hours away.

Right now, she's passing time, barefoot, in a trailer parked off bustling Madison Avenue in midtown Manhattan.

When she gets the call, she'll report for a 90-minute hair-and-makeup session in another trailer a block from here.

Then, returning, she'll slip into a white Thread Social eyelet mini-dress, floral-knit black tights and those T-strap pumps -- just the thing for a swank Sunday brunch, as staged by her new CW drama at the New York Palace Hotel up the street, where production trucks and crew have already converged.

It's about 7 a.m. on a bright, sunny July morning, and Meester is bright and sunny, too, having arrived a few minutes earlier wearing a lavender sundress and a smile, prepared to make herself at home until she's summoned.

White noise wheezing from the trailer's air conditioner does a pretty good job masking traffic outside, "but I keep realizing: I'm on the street in a box!" Meester laughs. "Like they say: The acting is for free. I get paid to wait."

True enough. But career-wise, Meester's wait could be just about over. Born in Marco Island, Fla., and raised here in New York before moving to Los Angeles, the 21-year-old actress got her start as a child model. She has appeared on series including "Shark," "House," "24" and "Entourage." Two seasons ago she was a regular on the short-lived NBC drama "Surface." She's made several films.

But now she's stepping into the high-profile, high-style role of Blair Waldorf, a scheming good girl who shares the luxe life with other fabulous Upper East Side prep-school teens. It's the sort of role, on the sort of show, that could make a household name out of Meester, along with her co-stars Penn Badgley (who plays Dan), Chace Crawford (Nate), Ed Westwick (Chuck) and Taylor Momsen (Jenny).

And don't forget Blake Lively, who plays Serena van der Woodsen, a notorious party girl now haunted by regrets. Serena is Blair's best friend and worst rival. How apt that, for the show's location shoot, Lively occupies the other half of this trailer. SERENA is inscribed on her door, just like BLAIR is on Meester's. It's the way things are on "Gossip Girl": Serena and Blair are close yet divided.

The series (which premieres Tuesday, September 18 on CTV) is based on the popular "Gossip Girl" young-adult novels. But its transition to TV is thanks to Josh Schwartz and Stephanie Savage, the creative team behind "The O.C."

Once again, they face a daunting challenge, which Schwartz, by phone from L.A., put this way: "How do you take a world that feels very exclusive and very rarefied, and make it universal? How do you make the viewer care about characters who seemingly have it all?"

If "The O.C." is any clue: By playing the exotic against the everyday; by striking a shrewd balance between what viewers have and what they pine for.

"I think the show will be relatable to lots of girls and even grown women," says Meester confidently. "And a guilty pleasure for guys!"

Could anything be more in synch with the zeitgeist? From her first "Gossip Girl" novel five years ago, author Cecily von Ziegesar presumed a Facebook/My Space/blogospheric culture that, only now, is coming true.

Here is a TV show whose narrator, like the novels', is an unidentified text-messaging guru. Gossip Girl, whoever she is, has her finger on the pulse of this trust-fund set -- the loves, ambitions, feuds and wretched excess. As voiced by Kristen Bell ("Veronica Mars"), Gossip Girl's blogging keeps everybody in the know, while keeping everything stirred up.

Never fear: GG has plenty of dish on B and S, as when, in the premiere, S pops up without notice after a year's mysterious absence. Where has she been? Why the self-imposed exile? Why did she resurface? And how will B feel about readmitting S into the fold?

It goes without saying that B is often tagged with another B-word. "But I can definitely relate to her," says Meester almost tenderly. "Blair's shortcomings are because of her insecurities. She feels all this pressure to be perfect: to be the most beautiful, the most popular, the most loved."

Serena seems forever winging it. But Blair struggles to follow a life-script she believes is already plotted out for her -- the right education, the right marriage (to boyfriend-since-kindergarten Nate), the right social circle -- while she keeps those insecurities hidden from sight.

"The only way to play Blair, or any character, and make her human, is to find what she is inside me," Meester says. "And I know I have my insecurities, too."

Does that make show business harder for her to navigate?

Sure, replies Meester, when you focus too much on your competition, which "can be so fierce and discouraging. But I think you can grow out of that, especially if you realize the only way you find success is: Figure out what works for you and what doesn't."

She laughs. "The way Blair and I are NOT alike when it comes to insecurities is: SHE pays SO much attention to hers!"

Blair is fated, it seems, to remain in Serena's shadow. Instant visual giveaway: Serena gets to be a dewy blonde, but Blair is moderated -- she's a brunette. (Meester, herself a blonde, was asked to go brown when she tried out for the role. "They said, `Please, you can't wear a wig.' So I dyed my hair before I even got the part. That's a big deal -- but I'm glad I did!")

Last March, Meester and her cast-mates shot the pilot (which will air as the series premiere), then waited to find out if they would be on the fall schedule.

A couple of months after that, she's back, to film episode two ... and beyond.

"There's butterflies, definitely," Meester says. "But it's a good kind of nervous energy, because everybody's really starting to get to know each other, and starting something new that we really hope works out."

Headquarters is at Silvercup Studios, the famed Queens, N.Y., lot where "Gossip Girl" shoots on the same stage occupied the past decade by "The Sopranos."

But the real New York City will be on display. Like the majestic front steps of the Metropolitan Museum of Art. The posh Henri Bendel department store. The Palace Hotel (which, according to the show's mythos, is owned by the filthy-rich parents of bad boy Chuck).

So it's fitting that viewers (or each viewer's cyber-surrogate, anyway) will have the chance to rub virtual elbows with "Gossip Girl" characters in the city they rule.

A "Gossip Girl" virtual world "will let the viewer walk down the streets where Blair and Serena walk, go to stores where they shop, go to concerts they attend," said Rick Haskins, the CW's head of marketing and brand strategy.

But the "Gossip Girl" Web site (where the virtual world should be online by the series' premiere) promises more.

"On each episode of the show," explained Haskins, "the story is pushed forward by Gossip Girl, and by the texts she sends to the characters' PDA devices. So we think this is the perfect show to dive deep into the digital realm."

To do that, said co-executive producer Savage, "We're always asking ourselves, `How can we supplement each week's 44 minutes of filmed programming with things on the Web that viewers will think are cool, and that connect with the story we're telling as organically as possible?'"

Maybe a viewer's online avatar can even chow down with the "Gossip Girl" gang. Say, at Sunday brunch?

"OK, ready," director Mark Piznarski calls out in the Palace Hotel's plush Madison Room, where, as this real-world July afternoon wears on, Meester and several of her co-stars (plus tony-looking extras) are gathered for the episode's big scene.

The cameras roll as Piznarski coaxes, "Havin' a good time at the brunch!"

And why not? There's caviar, mimosas, even models in Danskins hired to pose as marble statues.

But then two people show up Blair would rather not see.

"You have GOT to be kidding," she says with a withering smile.

Leighton Meester, poised to be a breakout star any moment now, knows a scene this rich was well worth the wait.