The production of Justice League has been troubled, to say the least. The tone has been lightened to distance the movie from the "grimdark" (side note: can we do away with this baby-talk term now?) affectation of Batman v Superman. The extensive reshoots saw some substantial changes made, such as the excision of Jesse Eisenberg‘s return as Lex Luthor, while Gal Gadot‘s Wonder Woman screentime was beefed up to capitalize on her picture's success.
The biggest issue has been the departure of Zack Snyder, who had to leave Justice League due to incredibly sad family issues, and was replaced by Joss Whedon (The Avengers). One look at any of the JL trailers instantly lets you know that the visual schematic is still 100% Snyder. However, Whedon apparently did enough punch-up to the screenplay that he earned himself a writing credit on the finished product, and early word indicates that the reshoots he performed added his own personal touches to the picture.
But how much of Justice League belongs to Snyder and how much belongs to Whedon? Producer Charles Roven weighed in with the AP, providing a numeric answer to that question:
“The goal is to make sure when you’re watching the movie, it all feels cohesive. That imprint that Joss had, some aspect of it is going to come out in the direction, but the actors are already pretty much down the road on their arcs. Let’s just say 80, 85 percent of the movie is what was originally shot. There’s only so much you can do with other 15, 20 percent of the movie.”
Obviously these are still rather arbitrary numbers without him going in and actually calculating screentime (if that's even possible on this type of Frankenstein movie), but it sounds like Whedon came in and batted clean-up while injecting his own trademark charm into the movie. With these sorts of rumors, only those involved with the actual production know the truth behind the tales, though we're undoubtedly going to hear many try and give kudos/assign blame for however Justice League turned out when it drops this coming Friday, November 17th.