During pre-production, the script is broken down into individual scenes and all the locations, props, cast members, costumes, special effects and visual effects are identified. An extremely detailed schedule is produced and arrangements are made for the necessary elements to be available to the film-makers at the appropriate times. Sets are constructed, the crew are hired, financial arrangements are put in place and a start date for the beginning of principal photography is set. At some point in pre-production there will be a read-through of the script which is usually attended by all cast members with speaking parts, all heads of departments, financiers, producers, publicists, and of course the director.
Even though the writer may still be working on it, the screenplay is generally page-locked and scene-numbered at the beginning of pre-production to avoid confusion. This means that even though additions and deletions may still be made, any particular scene will always fall on the same page and have the same scene number.