Hellboy himself will be voicing Sinestro on Green Lantern: the Animated Series.
Source: Showrunner Loves GREEN LANTERN's Deep Mythology
During an interview with Newsarama, showrunner Giancarlo Volpe revealed just who would be voicing the big villain (or currently greatest GL hero in comics) of Green Lantern. The character would be voiced by Hellboy himself, Ron Perlman. Volpe had this to say of how they got to finally use the character:
Volpe: Yes! We finally got permission to use him. I've heard folks call it an embargo, but it was never that official. The folks at DC Entertainment politely asked us to back off of Sinestro "for now" because they weren't really sure what his fate would be in the feature film [when we were in early development of the series]. They didn't know if his turn would be in the first one or later, and if you've seen the film they do hint at his turn to the Yellow Lanterns in the credits scene.
So seeing that gave us permission. It was great to finally get to put him in, because it doesn't feel quite like the Green Lantern universe unless he's in it. And I suppose I can tell you, we got Ron Perlman to play Sinestro! So, he lends a very interesting take and perspective on Sinestro's character. Plus that's a cool sci-fi fan gem there.
"The Prisoner of Sinestro" airs (we hope) January 19th.