Justice League - Details Regarding the Reshoot
Two months ago, it was announced that not Zack Snyder but Joss Whedon would be in charge of any kind of scheduled reshoots for Justice League because Snyder wanted to spend some time with his family after his daughter's death. Joss Whedon had already been involved in the production and he also wrote the new scenes.
Reshoots are not uncommon in Hollywood. They are usually part of the schedule from the very beginning and tae a few weeks to be finished. In this particular case, it takes a little longer though because some of the actors are already involved in other projects. Henry Cavill for instance is currently working on Mission: Impossible 6 which forces him to travel from one set to another. For his character in Mission: Impossible 6, he has a mustache and he can't shave it off for obvious reasons. Said mustache is supposed to be removed digitally for the new Justice League scenes.
Ezra Miller will be working on the sequel to Fantastic Beasts and Where to Find Them but since this is also a Warner Brothers production, it should not be that complicated.
The reason for the reshoots is simply: New dialogs are being shot in order to have a better transition for the particular sequences. All in all, the reshoots cost approx. $25 million.