Eustis appears right in the middle of the episode, at a rooftop gala thrown by Blake Lively's character Serena, who says some very nice things about the Public's recent summer revival of Shakespeare's As You Like It. According to a Public spokesperson, Eustis was invited to appear on the episode after Lively saw -- and did indeed enjoy -- that production.
The Public's head honcho isn't the only theater person of note to guest star on Gossip Girl. Charles Isherwood, theater critic of The New York Times, also made a brief appearance on the program in 2009.
Don't show this again.