Zack Snyder shot 5 hours of assembly footage during principle photography in 2016. From that, he edited it to 214 mins(3.5 hours) and was happy to call it his director’s cut. From this, he was happy to edit it down to 3 hours for the theatrical cut, and release the 3.5 hour directors cut in Blu-ray.
But WB wanted Zack Snyder to cut it to 2 hours for the theatrical cut. Which is unbelievable, since cutting 1.5 hours from a 3.5 hour movie would make it extremely unwatchable and make absolutely no sense. Snyder tried his best to negotiate with WB to release a longer cut, even made a 2.5 hour cut, which was extremely compromised and makes no sense, but WB wasn’t happy. This was when Snyder suffered a family tragedy and lost the will to fight with WB for the longer cut.
He stepped down and WB(Geoff Johns) used this opportunity to hire Joss Whedon, and use the 2 months of reshoots to reshoot almost the entire film. He wrote 88 pages of reshoots, which translates to almost 90 mins of the final movie.
The original cinematographer, Fabian Wagner, and later Snyder confirmed that only 30 mins of the theatrical cut of Justice League had shots by Zack Snyder, and even those were heavily edited. The rest were shot by Joss Whedon.
So Zack Snyder’s Justice League releasing next month, which is 4 hours, will contain at minimum 3.5 hours of footage we never saw.
The only new idea is the 4 mins of new footage he shot recently with Jared Leto and Joe Mangeniello, which he added since he wanted this universe’s Batman and Joker meet at least once. Other than that, it’s all shot in 2016.