Putting together a short film, a commercial, or a feature film isn't like stacking legos on top of each other- it's like putting together a 100-piece puzzle some some of the pieces hidden under the coffee table.
For a production to truly come together and be productive, everything must be executed from the writer's page, to the casting room, to the movie set, and then finally, on the editor's workstation. That's why it's incredibly important for casting directors, and writers, and directors, and editors to try and stick with the material that was originally agreed upon in order to achieve productivity.
Let's say you're at a casting facility auditioning actors in a casting room, and suddenly an actor comes in and reads for a part in a completely different way. Now there's a vibe in the casting room and people are wondering if changes should be made to the script in order to accommodate the actor's beautiful serendipitous performance.
Our advice? Only makes changes when it's necessary, or if the change doesn't throw a major monkey-wrench into the flow of the production so that it starts to effect every other aspect of the production.