Though the first Justice League film isn't set to start shooting until next spring, the script has been finished and handed into Warner Bros. since this past July, a whole 9 or 10 months ahead of filming at the very least. Granted, it's good to be prepared with a movie this important and insanely intricate, but it's fairly common in the fast-moving word of conveyor belt blockbusters for scripts to be continually hammered on up to shooting. Remember last year's Sony hack, where it became apparent that Spectre was still being massively reworked in the days and weeks before shooting, which may explain the underwhelming final product. Hopefully this indicates that Terrio and Warner Bros. are extremely confident with their story.