The 26-year-old actress sports a short wig and green costume as Peter Pan in the photo. Williams unveiled the picture Tuesday on Today, and said rehearsals have been "so fun."
"I can only imagine what the next couple of weeks and months are going to bring. I cannot wait," she shared. "We're so excited about the live part of it. I think that's what makes it so fun."
Father and NBC News anchor Brian Williams reported on his daughter's casting in July. Allison Williams has called the role "a dream come true," and declared "it's such an honor to be a part of this adventure."
Peter Pan was created by Scottish author J.M. Barrie, and first appeared in the 1902 novel The Little White Bird. The character starred in the 1904 play Peter Pan, The Boy Who Wouldn't Grow Up, and its 1911 book adaptation, Peter Pan and Wendy. Peter is a mischievous boy who wishes to never grow up, and adventures on the island of Neverland.
Peter Pan Live will air December 4 on NBC. The network's Sound of Music Live brought in 18.6 million viewers last December.