I'm a finicky reader. I can't read most fiction because it's either too boring (I'm constantly asking "So what?" after every sentence and looking at the page number to see where I am) or just plain poorly written at the most basic level. I try, periodically, to get through the first few pages of some new author's "breakthrough" novel—and invariably find the experience upsetting. Even if most fiction writers were masters of diction (which they aren't), I'd be disgusted by the sheer lack of storytelling craft evident in so many supposedly gripping novels. You wouldn't think it'd be that hard. A compelling character plus a decent plot line should result in a decent book, yes? No, actually. There's more to it than that. Way more.
Screenwriters, much more so than novelists, take story construction seriously. They know that a story is more than a few good characters with a plot that has a beginning, middle, and end. They know that a character can understand what he wants (or thinks he wants) without at all understanding what he needs, for example. Screenwriters tend to know (better than novelists) that character is best shown, through action, and not told through narrative or inner dialog. They understand the need for frequent plot reversals, people and events that aren't what they seem they are, crises that can't get any worse but do, the need for multiple plot lines that fold back on each other, etc. And they certainly understand, better than most novelists, the need for high-impact visuals.
All of which is by way of saying, if you're writing a novel, it might pay you to try writing the screenplay first.
A screenplay is an extended outline for a story, told visually and through dialog, containing a bare minimum of directorial advice. It runs 110 to 120 pages, properly formatted; never much more (nor much less) than that. It follows certain fairly rigid conventions as to style, yet obviously there's enough freedom within the constraints to allow art to happen.
Professional screenwriter Matt Bird gives a comprehensive checklist of things every story should try to accomplish in one of his best-ever blog posts. Matt's list runs to about 120 items. I agree with all of the items on the list; every single one of them can make a story better. If you can accomplish all 120 requirements in the scope of a 120-page screenplay, chances are you've got a halfway decent story, although you could still be a sucky screenwriter, of course.
I think if the average novelist were to distill his or her story into a screenplay first, before writing the novel, the novel could only be better, even if the screenplay version blows chunks. Sadly, novelists (especially first-timers) seem to think the rules of screenwriting don't apply to them. I think they're badly mistaken.
Do you need a film-school degree to write a decent novel? No, of course not. Can you learn something by writing your story as a screenplay? Yes. I think it may very well help you turn out a much-improved novel. It can't hurt to find out.
Resources
Scriptshadow's Advice Page has a roundup of well-worthwhile advice articles, centering on storytelling.
Matt Bird's Cockeyed Caravan is (for my money) the best screenwriting blog anywhere. Again, the emphasis is on basic storytelling, not the nuts and bolts of polishing a screenplay.
Screenwriting Tips, You Hack is Xander Bennett's compilation of pithy, on-the-money tips, mostly about storytelling. Not updated as frequently as I'd like, but the tips are timeless, and over 1,220 of them (so far) await you.
The Bitter Script Reader is the blog of an professional script reader, "that guy you need to get past at the agencies and production companies." Some of the better posts are buried in the archives, so be sure to allow plenty of time for browsing around.
Where to Download Scripts is Alex Epstein's curated list of online script sources.
