Chicago Manual of Style (18.2) has quite the exalted description of what it takes to be an indexer:

Who should index a work? The ideal indexer sees the work as a whole, understands the emphasis of the various parts and their relationship to the whole and knows—or guesses—what readers of the particular work are likely to look for and what headings they will think of. The indexer should be widely read, scrupulous in handling of detail, analytically minded, well acquainted with publishing practices, and capable of meeting almost impossible deadlines.

Professional indexers, I thank you for your efforts on behalf of all researchers.