"Zack sought out Joss," Affleck said, "And wanted him to be the guy to finish the movie when he wasn't able to. They definitely worked together to do something kind of special."
Producer Charles Roven said that Whedon was already involved with Justice League before taking over the reshoots, in addition to working on making his new Batgirl film.
"Zack decided that he wanted to do some additional photography … He thought that Joss' sensibility might be exactly right for what he was looking to do," said Roven. "So he showed Joss the film, and he talked about a number of scenes that we would be adding or expanding upon, and Joss wrote those scenes."