Nathan Fillion was the voice of Green Lantern in the Justice League: Doom cartoon/movie. He was also Green lantern (Hal Jordan) in the Green Lantern: Emerald Knights movie.

On the other hand, you have Tim Daly, who voiced Superman in Justice League: Doom, Superman/Batman: Apocalypse, Superman/Batman: Public Enemies, Superman: Braniac Attacks, and the Superman cartoon series of the late 90's. Unfortunately he was busy with the brilliant The Fugitive tv show, so he couldn't voice Superman in the Justice League cartoons.

So what happens when Tim Daly starts to go a little crazy? Call in fellow voice star Nathan Fillion! For you other old timers, Nathan was the lead character in the Serenity movie and Tim Daly was freakin' hilarious on Wings. Well, on to the show...!