Jay Hernandez of The Los Angeles Times has the story behind Ed Norton's involvement in "The Incredible Hulk" script.
When Norton came in to meet about starring as Banner in April, the film had already been greenlighted and there were just three months before shooting was scheduled to begin, just after Independence Day. But Norton had well-established (if underground) writing experience and strong ideas about how to separate the film from any confusion over its connection to the 2003 Ang Lee version by casting it in a more distinct, starting-over vein like "Batman Begins" or "Casino Royale."
So Norton's initial deal included payment not just for his acting services but for his writing talents too, with his draft contractually stipulated to be turned around in less than a month. As it turned out, Norton delayed work on another screenplay job to do "Hulk," and he continues to tweak the script as principal photography hits its halfway point outside Toronto.