Um, wouldn't they need a script before they start working on a cast?
No. When assembling a "package" movie, you frequently get the basic idea, the director and the stars lined up and THEN commission a script.
--Ted
Purely out of curiosity, can anyone name a few movies that were developed in this way?
Casablanca is the most famous example. The script was being written literally while filming was well under way, one scene at a time. No one knew how the story was going to end until the last page was shot. It's amazing how well the story holds together, as there was virtually no "story" at all to begin with.