Audience - if it's a universal story that can reach/touch a lot of people, write a screenplay. If the story is so niche that only a specific audience/community would enjoy it, maybe a novel is better. Appeal - Do you see this on the big screen and do you think it would do well? There's great stories and then there's great movies. Sometimes they're not the same thing. Length - If you can't see the story fit in under two hours, then maybe you're better off writing a novel. If it's episodic, maybe TV. That's all I can think of right now.

For me, it's all in the scope. Is it one person's single adventure (screenplay), or is it multiple adventures, or lots of characters sharing POV (novel)? Very basic way to tell: Do you have more than one shift in the hero's reality? A screenplay goes from home base to challenge/trial to new reality. Add a second new reality on top of that, and it's a novel. Hope this helps!

If you have any doubt it should be a screenplay, then it should be a book (or a comic book).

I had a story that started out as a novel. On the second draft I turned it into a screenplay. After I finished the draft I put it away, picked it up a year later and wrote it as a graphic novel. It remains unfinished, in any format, to this day. It's my Moby Dick...

Sometimes they seem to be suited to both and it would come down to which format you feel most comfortable working in. Mainstream commercial fiction seems well-suited to big screen adaptation because of the genre's (necessarily, by definition) wide appeal. Women's fiction is a massive market as well. Start getting into literary fiction and the film adaptations will likely have a much more limited appeal. Character-driven stories full of inner conflict are, IMO, usually better suited to media meant to be read rather than seen. Novels also generally allow for more complex storylines involving more characters. Considering time constraints for film, how much simplification of your story concept is going to be necessary? If you plot out your major points and on/around what page they would have to occur in a 100-page screenplay, and you think that's an impossible feat to accomplish without cutting too much from the story or eliminating too many characters, maybe a novel is a better approach. You should still pay sharp attention to your structure in a novel, but you'll have more room to stretch your legs. A screenplay will force you to get to the point and be efficient in your choices. Whether you want to do that is up to you. And, frankly, having looked at things from the fiction side before deciding to work on a screenplay, my impression is that the odds of succeeding as a first-timer are much better with a novel than with a screenplay.

If the character's emotional arc is played out physically, through what he does and what is done to him, it's a screenplay. If it's played out through conversations, or inside his head, it's a novel.

Thanks everyone for your comments. They generally align with my thoughts. At the end of the day, I would think one thing to do would be to sit in a quiet room and 'be' with your story for a half-hour or so. And conjure up your feelings about it. Then ask yourself, "Does this feel like a movie?" That is not to say a novel can't be a movie -- we see those adaptations all the time -- but rather if the story feels first and foremost like a movie, then write the screenplay. If you have any hesitation due to scope, audience, length, focus on interior world and all the rest, write it as a novel.

I used to think there was one right answer to this question. HBO's GAME OF THRONES got me into reading the books. I'm not sure which I like better. I'm also not sure which medium the story is better suited to. I think it may have more to do with the writer than anything else. While both mediums are words on a page, they convert those words into emotion in very different ways. Mastery in one does not equate to mastery of the other. I'm not sure there's a right answer to this. I can think up hundreds of examples supporting either argument.

This is an easy one. You've got less than 1% chance that your screenplay will ever be bought, and if so it's a less than 1% chance it will ever become a movie. Most screenplays just sit on the writer's shelf and even their friends won't read it because they don't understand what a screenplay is. If you write a book, you will get it published, even if you have to publish it yourself. Maybe only your friends will read it, but if they do, then suddenly your chances of selling a screenplay and getting a movie made increase exponentially. If you're lucky, you'll even get to write the screenplay adaptation. So you should write a book. However, if you plan to make the movie yourself and have the resources to do that, then write a screenplay. I know this sounds like bad advice on a screenwriting website, but you'll be happier.

Bryan, given the paradigm shift that has been occurring around eBooks, that is definitely a consideration - or should be - for every writer re every story they undertake. Of course, since 99% of books generate very little income, while a writer may have the satisfaction of being published, for most there's not much difference than that script sitting on a shelf. So if a story feels like it should be a screenplay, I would still argue to approach it as such. Besides a script that does not sell is still an asset. Someone can option it. It can get you representation. You can use it as a writing sample. And there's always a possibility down the road someone will produce it. Odds against it, yes, but it does happen.