A script supervisor, on any film set, is one of the most important crew members. The position requires a keen eye and accounts for the continuity in a film. Script supervisors make sure actors pick up that glass with the same hand each take or turn their head the right way. They also help the director to remember who needs to enter and exit the frame. Ultimately, this position is a combination of office work and on-set work and can make or break credibility of a film.
Read the script, inside and out, being sure to make a list of every action, hand gesture or movement throughout.
Make a script breakdown of everything that affects continuity, such as day or night, inside or outside, characters involved, wardrobe, items or props.
Discuss how stringent line continuity needs to be. Some directors do not care if some lines are said different. Others care VERY much. Communicate before making a decision on your own.
Develop a good chemistry with the director. Other than the director of photography, a script supervisor is the closest person to the director.
During the shoot, take notes on what the actors do differently each take.
Before a take, let the director know what an actor needs to do in terms of continuity.
Never speak to the actors unless the director gives permission. Though you need to make sure actors repeat actions, the only one speaking to them should be the director.