Joss Whedon gets writing credit on Justice League; Wrote at least 33% of story
He may have had more impact than we think.
The production of DC's upcoming Justice League film has been pretty crazy, to say the least. Before the release of Batman v Superman last March, Justice League was intended to be a two part movie with the first one ending on a cliffhanger. After the critical reception of BvS, Warner Bros. and director Zack Snyder scrapped that idea as they went into production.
In April of this year, director Zack Snyder had to remove himself from the movie after his daughter tragically committed suicide. The movie had already wrapped principal photography and was in the post-production stage but the planned reshoots had yet to happen. Snyder and WB selected The Avengers director Joss Whedon to finish up the movie and rewrite some scenes for the reshoots.
There have been various rumors suggesting that the movie's reshoots are really extensive and have changed things like the ending, the significance of some characters like Lex Luthor and Darkseid, and the overall plot of the movie. Some have even suggested that Joss Whedon has changed large portions of the movie. Star Ben Affleck also stated that it feels like the product of two directors' visions. Warner Bros. has obviously denied these rumors but we'll likely have no idea what's true until the movie releases.
However, we do have an idea of how much of an impact that Whedon has had on the project. Warner Bros. has officially given Whedon a writing credit on the movie even though he wasn't officially attached to the movie until just a few months ago after the movie had been shot. According to the Writer's Guild of America, to officially get a writing credit, the first writer has to write at least 33%. Any other writers need to contribute at least 50% and it has to make it into the final product.
This means Whedon needed to write at least 50% of the script unless it's considered an adapted screenplay, then he would have needed to write at least 33% of the script. We can assume that Joss Whedon had a pretty significant impact on the story and may have changed major pieces of the plot.
We'll just have to see how the final product turns out when Justice League releases on November 17th, 2017.