Mark Hamill has been an iconic figure in the comic book / pop culture world since the first time he bought a droid from a Jawa. From his turn at fighting the Dark Side, his time writing his own comic, becoming the definitive voice of the Joker or taking a two turns as the Trickster Hamill is beloved by and industry that he himself confesses to loving. So it makes perfect sense that the folks behind Free Comic Book Day would go to him to help promote the upcoming event.
Hamill does a wonderful job of promoting Free Comic Book Day and reminding people that advantages of going to a brick & mortar stores and being part of the comic book community. He also gets a plug in for his upcoming appearance on The Flash.
Free Comic Book Day is May 2nd and you can find your local participating retailer here.