Movies are normally shot out of order in small sequences called scenes. The script supervisor's primary duty during filming is to maintain continuity between takes so no logic errors show up on screen in the finished product.
Part of a script supervisor's job is to monitor basic scene elements such as wardrobe, makeup, and light and sound levels. They also watch actor placement within the scene.
In addition to their own duties, the script supervisor is tasked with overseeing the rest of the film crew to make sure their jobs get done as well.
The script supervisor takes digital pictures after takes to be included with written notes as shooting progresses.
In the editing room, the script supervisor functions as the eyes and opinion of the director when he isn't present.