"Kids will come up to me and say, 'Oh, I'm so excited for "The Killing Joke,"' and I say, 'Don't you watch that!'" the voice of The Joker told the crowd Friday at "Star Wars" Celebration Europe.
"The thing is, it's not for kids," Hamill explained, "and I really apologize for this. It's rated R [...] but it's really mean and nasty."
The "Star Wars" icon also reiterated that the film isn't a direct adaptation of the seminal 1988 comic by Alan Moore and Brian Bolland.
"And, by the way, for you Alan Moore fundamentalists, of which I am one, it's not -- because I thought of it as a book on tape where we were going to do every single word, right, and just do music and sound effects," Hamill said. "Because Alan Moore doesn't like anything that they do -- you know, 'V for Vendetta,' he didn't like 'Watchmen.' It's just, you know, he's a perfectionist, so he's going to hate 'Killing Joke.' But 'Killing Joke' is embedded in a larger story that I can tell you involves Batgirl and Batman. And, like I say, it's earned its R rating. I mean, they really did it like the book."
If that wasn't enough to sell fans, Hamill also delivered some lines from the film as The Joker.
Directed by Sam Liu and Bruce Timm from a script by Brian Azzarello, "The Killing Joke" also stars Kevin Conroy and Tara Strong. The film will premiere July 22 at Comic-Con International ahead of its one-night-only limited theatrical release on July 25. It will be available for Digital HD download on July 26, and ahead of its arrival on Blu-ray/DVD on Aug. 2.