For all intents and purposes, Superman might be invincible, but the script for Man of Steel is apparently not. Kurt Johnstad, who co-wrote 300 with Man of Steel director Zack Snyder, has been brought on to help rewrite David S. Goyer's (The Dark Knight) screenplay.
A source close to the project told Moviehole that some rewriting was necessary, but Goyer is too busy working on his draft of the new "Godzilla" movie (also for Legendary Pictures and Warner Bros.) to make the changes himself, so Snyder has turned to his go-to writer, which would be Johnstad. Apparently Johnstad's not the first to make changes, as producer Christopher Nolan's brother Jonah has also done some work on the screenplay.
There had previously been rumors that Goyer's script needed some work on the second act, but with Man of Steel so close to filming, Johnstad's most likely doing polishing work or something well within budget.