From IGN, Mark Hamill discusses voicing the Joker in the Batman: Arkham Asylum sequel:
But with The Joker forming such a large part of his acting career, it seems Hamill took some convincing to sign on for the sequel. "My answer to [developers Rocksteady] for the sequel was, 'Guys, we're never going to be able to top the original.' It was so claustrophobic. There were so many abilities like the stealth mode, and all those things you can do with the new technology. I wanted to be able to say I'd gone out on a high note."
So what changed his mind? "I got on the phone with Rocksteady and they really reassured me and told me what they were going to do with the sequel. But I'm sworn to secrecy!"
Nevertheless, there's plenty to be gauged from his final remarks, certainly about the fate of The Joker in this second part. "This will be my last, there's no question about that. But it's the last hurrah."
I always knew this day would come. I'm not going to nerd rage or even nerd mourning. He's been the voice of the Joker for eighteen years now. That's longer than Patrick Stewart's tenure playing Captain Picard (1987 to 2002, btw). Eventually it's time to move on, and he's getting to go out on his own terms here. What more could you ask for?