I do have another question now I've read today's post - what about if you're a historical writer and a contemporary document quotes a historical figure as saying something and you use that exact statement or quote in your novel - is that plagiarism? I'm thinking not but now I'm a bit worried - particularly since you mentioned historical writers!
Let's be clear about what plagiarism IS. It's lifting something someone else wrote and passing it off as your own.
If I copied the above email from one of my beloved Snarklings, posted it here, sans italics, quotes or other "notification punctuation"..THAT is plagiarism because it will lead you gentle readers to think it is the breathless prose of Miss Snark.
You can quote anyone you want SAFELY if you cite the source. If you read Miss Snark’s blog and quote her in your writing, you have to credit the blog and Miss Snark. (We take credit and debit cards of course...and cash is always nice).
Go back to the Chicago Manual of Style (not published by Vogue but oh well) and learn all those ibid, op cits, and citation styles. Miss Snark had them down cold when she was writhing ...errr.... writing her thesis on the Use of the Past Pluperfect In The Novels of Barbara Cartland, but sadly, now that her sheepskin is on the wall, she's lost every bit of style she ever had.
This poor lad whose book has been rescinded erred in not CITING the source rather than just using it.
Render unto Caesar that which is Caesar's (Matt 22:21) so to speak.