I'm going to tiptoe into spoiler territory for those that aren't overly familiar with Green Lantern, so turn back now if you're hoping for a Green Lantern film filled with surprises.
Parallax is Hal Jordan. So, does this mean Ryan Reynolds is playing both the hero and the villain of Green Lantern? Possibly. In the comics, Parallax was introduced by writer Ron Marz as Hal Jordan's new identity, a time-traveling DC Comics villain, when DC was only interested in promoting Kyle Rayner as the new, and only, Green Lantern. Later, writer Geoff Johns explained Parallax as a living alien entity that was the embodiment of fear, possessing Jordan against his will.
I'm not sure what will end up on screen -- Hal Jordan's evil alter ego or the big, yellow Parallax monster as seen in Johns' Green Lantern: Rebirth mini-series. It's an interesting twist in one of the most-anticipated movies of 2011.
(source: Comic Book Movie)