In the case of Hulk, after another writer's treatment was declined in early 2006, Marvel hired [Zak] Penn, who wrote three drafts over a year. By spring 2007, Penn was about to go off to promote his movie The Grand, but the studio and the director, Louis Leterrier (The Transporter), still felt that the screenplay needed work.
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 The Incredible Hulk and he continues to tweak the script as principal photography hits its halfway point outside Toronto.