Talk about opportunity knocking. JGL was not originally cast. Seth Rogen said in an interview that the film was 4 days into shooting when the original actor(name didn't register) had to leave the set for personal reasons and JGL answered the call.
As an aside, Rogen said in the same interview that during a recent meeting with Steven Spielberg and George Lucas, that Lucas was dead serious about the end of the world in 2012. On the other hand, Rogen stated that he has a license to buy medical marijuana in California, and he partakes heavily practically everyday. So maybe his memory of the conversation is a little clouded.