She wrote, "Thrilled to be playing Peter Pan live on NBC in December. I've been rehearsing for a REALLY long time."
When news broke that Williams landed the coveted title role in Peter Pan, "Girls" co-star Lena Dunham posted to Instagram, "Please welcome the delicious Allison Williams @aw to the 'gram. This is the time a puppy visited set and she got so excited she actually cried a little."
Follow Williams at @aw, and click here for more information on the live broadcast of Peter Pan.