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https://screenrant.com/xmen-dark-phoenix-delay-explained/
After originally thought to be arriving in theaters in November 2018, the release of the Dark Phoenix trailer confirmed a rescheduling to February 14th 2019. However, that date too has now been scrapped and the movie pushed back to June 2019. Reports of reshoots taking place muddied the waters further and led some to question whether or not there were fundamental problems behind the scenes causing these delays.
Kinberg has now explained the real reason for the date changes. In an interview with Collider (via CBR), Kingberg argued that reshoots are very much standard practice for Hollywood blockbusters now and roughly explained what parts of Dark Phoenix needed extra work.
“Mostly we focused on the third act, both emotionally and in terms of the physical scale and the action of the third act that we adjusted. And then there were certain things in the first two acts of the movie - very few things, but to set up those changes in the third act we had to adjust some things in the first two acts so that that all was fluid and felt consistent.”
https://screenrant.com/xmen-dark-phoenix-delay-explained/