Yes, yes and no.
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**Yes,** you're allowed to include [passing mentions][1] of a character as long as that person isn't substantially described or portrayed. For example, if I said (in the script) that someone was a "bit of a Miss Marple" (e.g., an amateur sleuth), I wouldn't need to pay a fee.

**Yes, you can use the character in [parody][2].** The entire premise of the "Scary Movie" series of films is that characters are presented in a comedic context, without the need to pay the rights-holder or seek their permission (although I suspect that permission was sought anyway, in order to avoid legal challenges).

**No, you [can't use a character that is presently copyrighted][3].** Introducing a character as "Miss Marple" and then having her, even as a background character,  solving crimes and speaking to the main cast would almost certainly fall foul of the Copyright Act. If you want to use an existing character in your own work, you'll need to speak to the rights-holder, in this case [Agatha Christie Limited][4]


  [1]: https://books.google.co.uk/books?id=f6RsBgAAQBAJ&pg=PA52&lpg=PA52&dq=%22mentioning%20a%20character%22%20and%20copyright&source=bl&ots=c6_mGrBYQG&sig=5cJOCLEuWWLGVqpaG-r0c_w4mzE&hl=en&sa=X&redir_esc=y#v=onepage&q=%22mentioning%20a%20character%22%20and%20copyright&f=false
  [2]: https://en.wikipedia.org/wiki/Fair_use
  [3]: http://www.nolo.com/legal-encyclopedia/protecting-fictional-characters-under-copyright-law.html
  [4]: http://www.agathachristie.com/about-agatha-christie-limited