"I sort of very tentatively offered to cut my hair," Williams, 26, told the morning show's co-hosts. "I was like, 'You know, if it's a thing that would help I could cut my hair,' and they were like, 'You're welcome to do that, we're still gonna put you in a wig.'"
In the pic, Williams wears Pan's traditional green garb as she sits atop a ship. The Girls star's long brunette tresses are tucked underneath the boyish wig.
Williams posted the image on Instagram following her live appearance and wrote: "The transformation begins! My first shoot as Pan."
On the Today show, Williams said playing Pan (in the adaptation of the 1954 Broadway musical) has been "so fun," while adding: "I can only imagine what the next couple of weeks and months are going to bring. I cannot wait."
Williams was announced in July to play the role of Pan. In the casting announcement, Williams gushed about her dream role. "I have wanted to play Peter Pan since I was about 3 years old, so this is a dream come true." She also joked on Instagram, alongside the same throwback pic of herself as child dressed as Pan: "Thrilled to be playing Peter Pan live on NBC in December. I've been rehearsing for a REALLY long time."
Taking on the role originated by Mary Martin over six decades ago, Williams will play Pan opposite Christopher Walken who will star as villain Captain Hook. NBC's Peter Pan Live! will air on the network Dec. 4.
Sign up now for the Us Weekly newsletter to get breaking celebrity news, hot pics and more delivered straight to your inbox!