It’s a beautiful, busy afternoon in Neverland. The emerald-green fiberglass lagoon is glistening. The synthetic rosebush trees are in full bloom, and the scent of freshly cut lumber wafts through Stage 1 at Grumman Studios on Long Island.

It’s only been a day since the company mounting NBC’s “Peter Pan Live” musical moved onto the stages in Bethpage, N.Y., where the live telecast will originate on Dec. 4. With three weeks until showtime for this one-night-only affair, crew members are swarming the elaborate sets, pounding nails into the Jolly -Roger, re-arranging the furniture in the Darlings’ London house and adding decorative touches to the Lost Boys’ hideout.

Even in this nascent state, there’s an air of audacity to the undertaking that is energizing to all involved. On a site where Grumman Corp. once built airplanes and fighter jets, Peter Pan is poised to take flight in a three-hour extravaganza that is NBC’s follow-up to last year’s smash “The Sound of Music Live.”

“It’s a little scary,” admits Christopher Walken, who plays Captain Hook in the tuner based on J.M. Barrie’s fantasy-adventure play. “There are so many people working on it, it’s like some sort of military operation.”

The “Peter Pan” army consists of 46 cast members and another 350 or so crew members and production staffers, led by exec producers Craig Zadan and Neil Meron. On the big night, 16 cameras will be moving all over three primary sets to capture the action under the direction of Broadway vet Rob Ashford and seasoned live-TV hand Glenn Weiss.

It takes that many people and a budget estimated at $10 million to bring “Peter Pan” to life, because they’re essentially producing two shows at the same time. At its core, the show is a stage musical, but one that will be delivered to a television audience of millions (so NBC hopes) rather than a crowd of hundreds in a theater setting. That means the choreography of the camera work, and the effects that will supply a CG Tinker Bell (an image that will be manually manipulated in real time by a tech) are getting almost as much rehearsal time as the dance numbers and the sword fights.

Meron promises that “Peter Pan” will have a grander, more film-like feel than did “Sound of Music,” because it’s a more expansive production overall. “Sound of Music’s” sets were laid out in one long line along a proscenium. With “Pan,” there’s more derring-do across multiple stages. Not to mention the flying. “It’s a bigger show than ‘Sound of Music,’ ” Meron says. “It’s a true balls-out musical.”

Nobody is under more pressure than Allison Williams, the “Girls” star who will take wing in the title role, continuing the legit tradition of women playing the feisty boy who refuses to grow up.

For five months, she’s been undergoing an intense regimen of flying lessons (courtesy of the legit institution Flying by Foy, which trained all the actors who will take flight), physical training, a strength-building diet, singing-and-dancing workouts and sleeping with ice packs “in places where it’s not very ladylike to have an ice pack,” she notes. No matter how much padding is put on, the harness that she wears to fly — which could easily double as a chastity belt — chafes until calluses are built up, she adds. And she has suffered plenty of bumps and bruises while learning to hit her marks in midair, waving off scrapes and scratches that probably would have brought the medic for the character she plays on “Girls.”

Still, Williams can’t believe her good fortune. “This is all you ever want as an actor,” she says during a rehearsal break on Nov. 12. “All you want is for someone to ask you to work very hard and push yourself — and tell you that ‘We have faith in you to do it.’ ”

Faith is what it took last year for NBC to roll the dice on “Sound of Music Live.” NBC Entertainment chairman Bob Greenblatt had a hunch that a splashy live showcase of a beloved musical airing at the start of the December holidays would draw a family crowd.

Greenblatt has experience in musical theater — he won a Tony this year as a producer of “A Gentleman’s Guide to Love and Murder” — and in Zadan and Meron, he knew he had a pair of pros who could pull it off. They initially had their eye on “Oklahoma,” but eventually decided that “Sound of Music” was more familiar to general audiences, thanks to the 1965 Julie Andrews feature.

The 22 million viewers who heard the hills come alive last Dec. 5 proved to be a rocket booster for the Peacock’s ratings turnaround. Auds were not only exposed to promos for new NBC shows, but the social media chatter was (mostly) celebratory — a welcome change for a network that had long been the butt of jokes.

This year, the core team that pulled off “Sound of Music” has reunited for “Peter Pan.” And just like last year, Zadan and Meron are also in the nitty-gritty of producing another live TV spectacle, the Feb. 22 Oscarcast for ABC.

“This is a real producing job,” Meron says, noting the “excavation” work that went into sprucing up “Pan’s” book and song selections. Zadan and Meron have no doubt that the timelessness of the Peter Pan character and the promise of seeing Williams take flight will attract the family audience NBC has promised to its premium sponsors. But the musical itself was in need of an overhaul.

“In the cold light of day, ‘Peter Pan’ isn’t the bona fide classic that ‘Sound of Music’ is. It’s not one of those shows that you just don’t touch,” Meron says. “That gave us license to re-examine the music and the role of Hook. We were able to strengthen and deepen that role, and add our own touches throughout.”

The first step was to tap screenwriter Irene Mecchi, whose credits include “The Lion King” and “Brave,” to revise the book, adding more drama and more menace for the Hook character. Then they sifted through the tunes penned by the original team — Jule Styne and Moose Charlap and lyricists Carolyn Leigh, Betty Comden and Adolph Green — and found some material that had been cut from the Mary Martin rendition or added to road versions over the years.

Amanda Green, daughter of Adolph, was enlisted to help flesh out some material. One song that’s been restored with new lyrics is “When I Went Home,” a melancholy tune “that makes you understand more about Peter,” Meron says. There was also a major reworking of the song “Ugg-a-Wugg” that could have been viewed as offensive by Native Americans; the new version is titled “True Blood Brothers.”

Musical director David Chase, who also worked on “Sound of Music Live,” had the hard task of weaving in the new material and overseeing new arrangements.

Production designer Derek McLane, another “Sound of Music Live” alum, was let loose in April to conceive the tuner’s look. From Peter Pan’s deep green duds to Captain Hook’s burgundy velvet coat, the palette aims to reflect the show’s magical fantasy nature. Neverland in particular is a visual feast of candy-colored props, including those giant rosebush trees, which look like something straight out of Oz.

“Neverland is laid out like a treasure map,” McLane explains. “Within that, we wanted to offer a version of nature that is presented in a very unnatural way.”

As if there isn’t enough pressure in mounting a stage musical for live TV, the “Peter Pan” company knows the success of “Sound of Music” raises the bar even higher. Perhaps to acknowledge the daunting task at hand, amid the hustle and bustle of rehearsals, there’s a strong sense of camaraderie on the set.

Christian Borle, another “Sound of Music” alum who limns the roles of both Mr. Darling and Smee in “Peter Pan,” is clearly having fun playing pirates with Walken during a rehearsal on the Jolly Roger deck. Crew members working in other areas stop to watch Walken sparring and soft-shoeing. Williams says she’s gotten an education in working with her co-star, whom she describes as an actor of formidable technical prowess. Walken, in turn, has been impressed by Williams’ range at this early stage of her career. “She’s a great singer,” he says.

Everyone involved is all too aware that the clock is ticking, and they have only one shot to deliver their best work.

“This is quite different from anything I’ve ever done before,” Walken says. “When you do a (stage) show, even if it isn’t a hit, you’re in previews for a few weeks and you get comfortable in the role. This is rehearsed as a stage show, but then the cameras are there. I’m never even sure when the camera is on me.”

More than a few of the key “Pan” players expressed gratitude to NBC’s Greenblatt- for championing musical theater on TV. -“All of the networks are searching for things that will drive big audiences,” Meron says. “With Bob, you’ve got someone with good taste who is not afraid to try something new and risky.”

Williams channels the spirit of her character in expressing hope that viewers suspend cynicism, and tune in with the perspective that “you have to be a believer in magic.” No matter what happens on Dec. 4, she feels she’ll never be the same after her time in Neverland.

“This is changing my life,” she says. “This is what I was waiting for as an actor, only I didn’t know to dream this big.”