When Allison Williams swoops into the Darling family nursery in NBC’s forthcoming live broadcast of Peter Pan Live!, she might flash back to the first time she donned Peter’s grass-green tunic — more than 20 years ago, as a preschooler on Halloween. Williams’ lifelong love of the boy who won’t grow up comes full circle on December 4, when the 26-year-old actress takes flight across millions of TV screens opposite Christopher Walken as Captain Hook.

During a break from her rigorous rehearsal schedule, the poised and friendly Williams recalled “watching the VHS tape of Mary Martin and Cyril Ritchard on repeat” as a kid in New Canaan, Connecticut. “I just loved the energy of it,” she says of the 1960 TV special, which preserved Martin’s Tony-winning performance. Young Allison especially embraced the notion of a make-believe world “where if you have a big imagination, you win. That’s still what I love about it.” The moment NBC announced plans to follow up its highly rated 2013 live presentation of The Sound of Music Live! with Peter Pan Live!, Williams set her sights on the title role.

Created by J.M. Barrie in a 1902 novel, elfin Peter and his band of Lost Boys took the stage for the first time two years later. (In keeping with British pantomime tradition, the character was played by a woman.) The musical version, created especially for Mary Martin, debuted on Broadway in 1954, with a score by Moose Charlap and Carolyn Leigh and additional songs by Jule Styne, Betty Comden, and Adolph Green. The show had been crafted with television in mind: Martin’s original run was cut short for a live 1955 telecast on NBC, and she did it again in 1956 and 1960. By then, the star was in her late-forties, but no one cared: Audiences shared her view that “Neverland is the way I would like real life to be — timeless, free, mischievous, filled with gaiety, tenderness, and magic.”

In choosing Williams to headline the 2014 telecast, producers Neil Meron and Craig Zadan departed from the typical casting of petite blondes with pixie haircuts. (Sandy Duncan and Olympic gymnast Cathy Rigby followed Martin to Broadway, receiving best actress Tony nominations in 1980 and 1991, respectively.) The brunette Williams, best known for playing ultrafeminine Marnie Michaels in the HBO comedy Girls, has become a dazzling red carpet presence in ball gowns by Ralph Lauren, Dolce & Gabbana, and Oscar de la Renta. But in Peter Pan Live!, she’ll sing “I’ve Gotta Crow” in a boyish wig and no makeup, an assignment she insists is not really a shocking departure.

“I was a tomboy, absolutely,” she says with a laugh. “I grew up playing a ton of sports and always loved hanging with the guys. Now I’m surrounded by a cast of guys, which is so much fun. I’m rehearsing in sneakers or these big, brown boots Peter wears; there’s not a pair of heels in sight. I’m living a luxurious life in leisurewear.”

Singing also comes naturally to Williams, who grew up going to Broadway shows including The Phantom of the Opera and South Pacific (starring Kelli O’Hara, who will play Mrs. Darling in the live broadcast). “My parents always knew I was going to be an actress,” she says of NBC news anchor Brian Williams and his wife, Jane, “but they insisted I go to college first. Initially I pushed back, but they made a good call. I don’t think I would have been able to handle the rejection of being a child actor.” Shortly after she graduated from Yale, a YouTube video of Williams singing Nat King Cole’s “Nature Boy” to the theme from Mad Men caught the attention of Judd Apatow. The producer cast her in Girls, which begins its fourth season early next year.

“It’s the funniest juxtaposition,” she says of dubbing audio in the morning for a sex scene on Girls and chatting with Tinker Bell in the afternoon. “Actors dream of having two jobs that are this different. [As Peter] it’s a treat to behave in such an energetic, adventurous, uninhibited way, and it makes me love playing Marnie even more.” Next March, Williams will show off even more of her vocal range in a solo concert of tunes from “Broadway, Disney, pop, country, folk, everything!” as part of Lincoln Center’s American Songbook series.

Prepping for her musical debut in Peter Pan Live! involves more than strapping on a harness and soaring around a soundstage. Williams ticks off a three-month regimen of “vocal lessons, physical training, flying lessons, fight choreography lessons, dialect lessons — it was like being in a conservatory, learning something new every day.” She’s thrilled to be surrounded by Broadway veterans while performing Tony winner Rob Ashford’s choreography. “All of my Lost Boys have been in the business forever,” she says, not to mention costars Christopher Walken, Kelli O’Hara, and Sound of Music vet Christian Borle — a Tony winner, coincidentally, for playing a Captain Hook–inspired character in the Peter Pan origin story Peter and the Starcatcher.

Working with Oscar winner Walken “is like being in a master class,” Williams says. “His delivery of his lines changes every time, and it’s a great choice every time. Kelli O’Hara has the voice I aspire to have, so angelic and precise and gorgeous. And Christian is so deeply funny as Smee and so funny and human as Mr. Darling. I often look over and can’t believe I am working with them.”

Williams’ modesty is balanced by a quiet confidence that she is prepared for the demands of starring in Peter Pan Live!. “I’m aware that not everyone was excited when I was announced,” she says, adding with a laugh, “In today’s culture, there’s no such thing as a unanimous win. There are even people alive who don’t like Beyoncé! But what everyone can count on is that I will be respectful and continue the long tradition of actors playing this character. I’m having the time of my life, but I’m also taking it very seriously.”

In a joint Today show appearance, Sandy Duncan and Cathy Rigby urged Williams to reprise her performance on stage, an idea that intrigues her. “They said that I’m not going to be satisfied by a single live show,” she recalls. “Of course, I don’t know how or where or who would pay for [a theatrical run], but it is a shame we only get to do this once! The main thing they told me was just to have fun. They miss playing Peter, and the minute it’s over, I feel sure I will too.”