I gather people are wondering about the overlapping issues of copyright infringement and plagiarism in writing.
Copyright infringement is the unauthorized copying and use of a significant portion of another authors' words.
Plagiarism is the fraudulent misrepresentation that the words, research, ideas, etc. in your work are yours alone and that you didn't borrow (or steal) them from other authors or researchers.
Clearly, the two can overlap, but copyright infringement is the harder tort to prove in court, in part because fewer elements of another's work are copyrightable (the words are but the ideas and research aren't) and in part because the borrower has some defenses (fair use, parody, etc.) to the claim that he's infringed the original author's copyright.
Plagiarism is more of an ethical matter than a legal one. It's wrong to claim as your own the work of another, but it may not be illegal as a civil matter. "Work" in this context includes all sorts of stuff: research, original or unique ideas, characters, even terribly clever plots. As with copyright infringement, the more original or unique the source material, the less likely it is that the borrower came up with it on her own.
Characters are interesting in this regard. If I write a book that has an Irish Setter named "Ron Weasley" in it, J.K. Rowling's lawyers won't be coming after me. If I write a book with a character named "Ron Weasley" who's a wizard, has four older brothers, a younger sister, and a best friend named Harry -- I should expect a very curt letter. The devil's in the details -- the more of J.K. Rowling's details I put in, the more likely it is I'm infringing her copyright in the character(s) she's created.
(I consulted Henry before writing this post -- he's an intellectual property lawyer, so of course I did -- and he pointed out that Rowling may have trademarks in all her major characters, so that even using the name Weasley on a red-haired character could be a problem. Not sure if my Irish Setter will survive a trademark dispute, in that case.)
Let's look at Eloisa James' book When Beauty Tamed The Beast. She's been quite open about how she based her hero, Piers Yelverton, Earl of Marchant, on Hugh Laurie's character from the show. Is that infringement? Plagiarism? Or something else?
First off, I've read that she had explicit permission of the producers of the show House to do...something. I'm equivocating not merely because I don't know the parameters of their agreement, but also because I have a hard time imagining that any author would need anyone's permission to create a character in a historical romance set in England who's based on Dr. Gregory House (an modern day American expert diagnostician) who's based on Sherlock Holmes (a late Victorian English consulting detective) who's based on Edgar Allan Poe's C. Auguste Dupin (a Frenchman who uses ratiocination to solve crimes in the mid-19th century).
Eloisa James didn't need anyone's permission to create a character that could be described as "the "arrogant know-it-all" any more than Chuck Lorre needed someone's permission to create Dr. Sheldon Cooper, his "arrogant know-it-all" physicist in The Big Bang Theory. My guess is she got permission, in effect, to tell people that her hero in When Beauty Tamed The Beast was based on House. (There are some very complicated rules on using another person, or in this case character, to promote your own product. Making money off someone else's intellectual property is always risky.)
Giving credit to another author is a defense to claims of plagiarism. Hitching your story to another character's popularity, depending on how commercial that hitching becomes (answering an interview question is one thing, taking out print ads trumpeting the connection is another) is no longer an issue of giving credit where credit is due.
I don't know if this clears anything up, so post further questions in the comments and I'll get Henry to fill in all the blanks.