Quality Control makes sure that things are done correctly and things are consistent from page to page.
They make sure everything is typeset correctly, spelled correctly, and grammatically accurate according to the script. They also make sure that things like blacks are done correctly, whites are cleaned thoroughly the grays are done consistently with the same pattern or shade. They make sure that borders are consistent, aligned, and same size.
You have to have good eyes, and if you're in the first stage of QC where you actually fix the problems, then you have to be a good cleaner.
Well, it really depends on the group how much the QC actually does. But there is no basic script. The QC has to do an in-depth of a page, and if qualified they'll fix the problem. In TMI the qualified cleaners will go through QC and fix the problems, then they're passed onto the chapter QC who goes through and looks through the final product, typesetting and all, and search for more problems.