The Fight Club actor agreed to star in the comic book adaptation on the proviso that he was allowed to write the script.
Norton reworked the screenplay but was only able to change dialogue and develop characters because much of the film, including the opening sequence in Brazil, had already been prepared.
Marvel submitted both Norton and Penn's draft to the Writers' Guild of America (WGA), which awarded Penn sole credit after reading his plea. The WGA reportedly favoured Penn's original plot structure and characters over Norton's dialogue changes.
Director Louis Letterier admitted that there had been several disagreements between Norton and filmmakers.
He told Entertainment Weekly: "Everyone was exhausted; it was like a little burst. 'I'm angry with you!' 'No, I’m angry with you!' And me in the centre saying 'Boys, calm down'. It didn’t come to blows. It was just a remark here and there, then it became public."