Hamill, best known for his role as Luke Skywalker in the original Star Wars trilogy, decided to surprise those in attendance by reading a few lines from his upcoming role in Batman: The Killing Joke in his iconic Joker voice.
"Memory's so treacherous," he said in his ominous Joker voice. "One moment you're lost in a carnival of delights, with the poignant childhood aromas flashing to the yawn of puberty. All that sentimental candy floss. Next, it leads you somewhere you don't want to go, somewhere dark, and cold, and filled with the damp, ambiguous shapes of things you hoped were forgotten. Memories can be vile, repulsive little growths. Like children, I suppose."
ET's Ashley Crossan was at the panel, and captured Hamill's impromptu performance.