Actors vary as much as directors. She is available to run lines with actors between takes, to read off-camera dialogue, and to write down ad libs and changes in dialogue so they can be matched in reverse shots and additional coverage. The script supervisor also helps the actors match themselves from shot to shot. As with all the film crafts, script supervision demands a personality suited to the work. Being observant, quiet, organized, a little on the obsessive side, able to assert yourself when needed, will all serve a script supervisor well.
Several options are available to learn the craft.
Courses at schools such as AFI, UCLA Extension, or the Vancouver Film School give a good grounding in theory. Of course, the best way to learn is to do. Offer to work for free on small projects. Along with knowing your own craft you should know about movies, film history, styles of coverage, editing, and acting. In the midst of that chaotic process which is filmmaking, a script supervisor contributes precision and accuracy.