WHAT THE HECK IS A SCRIPT SUPERVISOR?
Disclaimer: This information is based on Eve Butterly's own two decades+ of experience in the film business. You are free to disagree with her.
The short version for non-film people:
A Script Supervisor takes written notes for the editor to use in post production (editing), and keeps track of details on the set to ensure continuity of action during subsequent shooting.
Now... aren't you afraid you asked?
A Script Supervisor works closely with the Director to ensure that all necessary shots are covered, and that they will all edit together seamlessly: that is, there are no glaring errors in screen direction, eye lines, continuing action, etc. Also provides emotional support, sticks of gum, pieces of scratch paper and "poor babies!" if needed.
He or she works with the Assistant Directors to guarantee coverage, adjust the shooting schedule, and to fill out production reports at the end of the day.
The Camera Department consults with the Script Supervisor to slate each scene correctly, to make their logs and reports agree, and to recreate focal lengths and filters for matching shots. The Sound Mixer also needs slate and circle take information.
Scripty also works closely with other Department Heads on the set to ensure continuity of hair, make up, props, wardrobe, set dressing, picture cars, and so on.
He or she works with the Actors by running lines with them, reminding them of previous action (which hand they used to pick up an object, for example), and by revising the script to match the actors' ad libs or mistakes.
The script reports and logs are used by the Production Office to communicate the progress of the filming: how many scenes were completed today? What time was the first shot of the day? Why did we shoot so much coverage on scene 24? This information is closely regarded by the UPM, studio execs, payroll accountants, investors and/or the bond company.
Last, but not least: the Script Supervisor's notes are the bible that the Editor uses to cut together a project. Editing time is precious, not to mention expensive, and any way to cut down on fruitless searching for footage is a good thing. The notes tell the Editor what each shot consists of, which scene in the script it refers to, the camera and sound rolls it is recorded on, which takes are complete and usable—and which takes are not, and why.