Um, wouldn't they need a script before they start working on a cast?
The Hollywood film industry isn't really script-driven. Scripts are seen merely as raw materials to build with, and screenwriters merely as hired contractors whose job is to give the directors and producers what they ask for. A script may get a studio interested in doing a project, but then the original script will be put through countless rewrites, other writers will be brought in, the director and producers will often pick and choose pieces from multiple different scripts and mash them together, and there's plenty of rewriting or improvising on the set. And when it's a pre-existing franchise, there's no need for a pre-existing script to get the studio interested in making a movie. All they need is a concept and some stars, and whoever gets hired to do the script will then be given their marching orders as to what characters the script will be about and what direction to take the story in.