Variety helps clarify some of the details
Filming on “Justice League” had already finished, and Snyder was in the throes of post-production in order to meet the film’s Nov. 17 release date. Whedon will now oversee a handful of reshoots that had already been scheduled prior to Snyder’s daughter’s death, as well as the post-production process.
and from
Hollywood Reporter,
Snyder, after screening a rough cut of Justice League for fellow filmmakers and friends, wanted to add additional scenes, so he brought Whedon on board to write them. But as he prepared to shoot the scenes in England, Snyder realized it was not the time to leave home. “The directing is minimal and it has to adhere to the style and tone and the template that Zack set,” says Emmerich. “We’re not introducing any new characters. It’s the same characters in some new scenes. He’s handing the baton to Joss, but the course has really been set by Zack. I still believe that despite this tragedy, we’ll still end up with a great movie.”