I got this email the other day:

Suppose you adapt something that you love (a novel or a comic or short story) and it turns out very good. Would it be ethical to use this as a writing sample? Would it be a good idea? Would it demonstrate to a producer or agent your ability to adapt other materials?

On the one hand this seems to me very much like fanfic in that you’re using characters and a world created by some one to which you have no rights. On the other hand when you spec a TV show, which you do use as a writing sample, you’re doing the exact same thing.

I have to admit that these are questions nobody has ever asked me before.  The answer is no, you should not adapt someone else’s novel for your spec script.  And here’s why:

1) The point of a spec feature is to show off your unique voice and your ability to tell a compelling, original story. No producer is going to be impressed if you adapt THE DAVINCI CODE as your spec.

2) The point of an episodic spec is to show your ability to capture the structure, voice, characters, and tone of an existing TV series.  Basing your spec on a book, comic, or pre-existing movie tells a producer absolutely nothing about your grasp of the four-act structure or your ability to mimick the voice of a TV character.

3) You don’t own the book, comic book, or short story. It’s not yours to adapt. It’s stealing.

4) It’s not even remotely the same thing as writing a spec episode of an existing TV series. It’s accepted practice within the TV industry that it’s okay to write an episode of an existing series for the sole purpose of using it as a writing sample. You’re given a free pass, essentially, to play with characters you don’t own because there’s an understanding you’re not going to publish it, produce it, or sell it. A spec episodic script is a sample of your work, a way for producers to gauge if you can mimic the plotting, voice, structure, and tone of a TV series.

5) It’s an enormous cheat. Let’s be honest, you’re turning to a book, comic book, or other pre-existing property
because you’re too lazy to do the work involved in coming up with an
original story. Or you don’t have the skills to mimic an episode of a TV show. Or you’re so blinded by fanboy love of the material that
you can’t see what a stupid idea it is to send out your own adaptation.
Here are some of the least offensive things agents and producers will
think of you if you send out your unsolicited adaptation of  CATCHER IN
THE RYE or CAPTAIN MARVEL or your reimagining of SEAQUEST DSV:  "Loser,"
"geek," "never been laid," "speaks fluent Klingon," "talentless
amateur," "moron," "loves Real Person Slash Fic," "Collects unicorn
statuettes,"  "Lives with his Mother," "Dimwit," and "Longs for the return
of the original BATTLESTAR GALACTICA."

All that said, I vaguely recall reading somewhere that John Irving gave a young director the rights to one of his books (perhaps A WIDOW FOR ONE YEAR or OWEN MEANY) based on an  screenplay adaptation of the novel that the film-maker wrote on spec to impress the novelist.  But that’s a unique situation and very different from what you’re proposing.