The author of a manuscript is responsible for the accuracy of direct quotations from printed matter, interviews, and speeches. Rarely are copyeditors provided with the original documents and asked to verify (or spot-check) the word-by-word accuracy of quotations in a manuscript, although well-known quotations can be quickly verified in Bartlett's Familiar Quotations or a similar compendium.
Whenever a direct quotation appears in a manuscript, copyeditors are expected to
query or correct any obvious spelling errors in the quotation
call to the author's attention any odd wording within the quotation that suggests that words were mistyped, deleted, or otherwise miscopied
enforce consistency in deciding which quotations are run into the text and which quotations are set off as extracts (block quotations)
make sure that opening quotation marks have closing mates and that quotation marks within quotation marks are handled correctly
make sure that the syntax of the quoted matter fits the surrounding text
mark ellipsis points correctly and delete unnecessary ellipsis points
ensure that the quotation is attributed to its source
All the major style manuals offer extensive guidelines on these issues; here, we will survey the principal points.