One of my first real jobs was working as an assistant to a illustrator who did movie posters for Disney. I saw firsthand the process that he went through with the studio to get the exact poster they wanted. There are several concept posters that are created before the studio picks the one they want to go with.
I have several pieces of unused poster art here from John Alvin, who is responsible for creating over 130 movie posters over the course of his career. The designs you see here feature posters for Jurassic Park, Tim Burton's Batman, and Joel Schumacher's Batman Forever.