One of the main questions I always get asked on set is 'what does a script supervisor do?'. There seems to be a misconception that script supervisors are like the script editors. I once asked a student director if he needed a script supervisor, and the reply was a confused look and 'I should be fine, the script is finished and has been looked at'.
As a Script Supervisor starting out, I've come to liken the role to a bridge that links the script to the footage. If you imagine giving the current shooting script to the editor, along with the filmed footage; no knowledge of how long the film will be, no idea what shots went well, what line changes were made which would effect the order of the shots. You can imagine, the editor would have a hard time. When editing student films, this was rarely an issue because the footage was so little and the stakes were so low (losing a few marks does not compare to losing a few million pound).
So, there are four main things a script supervisor does. Directors may want you to do less, and more than these four:
A script is given to the script supervisor who times and reads each scene. And so predicts the running time of the overall film. Whilst on set the script supervisor times the actual footage, so they would then know if the film was overrunning or running short. This gives the director food for thought as they could, while filming, already be thinking of ways to cut time, or make it up.
The script supervisor writes a log of every shot, stating the take, lens, shot, action recorded, and most importantly if that take was bad or good. If it was bad, a note would be placed in the notes about what was wrong. This makes it easier for the editor to sift through the takes.
SCRIPT CHANGES, DISCREPANCIES, AND CONTINUITY
I've placed these under the same title because they are all to deal with how the current script translates to the footage. Any changes to the script made on set or during a shoot by the actors has to be noted for the editor. Any changes made by the actors through improvisation on camera, also has to be mentioned to the director. The changes must stay the same on every take...
...which leads us on to the thing we are most known for- continuity. Making sure the action in one shot matches the others exactly, so it cuts well in the editing suite. For example, it would look weird if aketchup bottle was on the table in one shot and gone in the next. Same with the script overall, if a person lost their sock down a well in scene 4, and scene 5 straight happens after, he cannot wear two socks in that scene.
Any discrepancies also have to be given to the director. This happens immediately after the script supervisor takes the script. It is prove- read and notes are taken of anything that doesn't make sense. For example, if in scene 4 the character reveals they are a vegetarian, but in scene 8 they are eating a rare sirloin steak (unless it's part of a big reveal).
It is useful to know what parts have been shot and what hasn't. As a student, it was fine just ticking scenes off as you go along. But, with a fair few set ups it's easy to forget what lines have been spoken on screen and by what angles. Many tend to use tramlines. This is an example: