For the legitimately curious, I'd recommend starting with anyone from the ancient Greeks to Chekhov. For more basic, and perhaps more practical guidelines, I'd recommend anything by Syd Field, William Goldman, Robert McKee, or J. Michael Straczynski. Those are sort of the time-honored classics in the how-to genre of screenwriting and story structure.

A few tidbits, and by no means comprehensive:

- Typical screenplays weigh in at around 90 to 120 pages, and most of the time, execs prefer a screenplay to hover around the southern end of that spectrum.

- A screenplay usually follows a three-act structure. If we take a 90-page (roughly 90-minute) screenplay as a template, then the first act is the first 30 pages. The second act occurs from roughly pages 30 to 60, and the third act roughly from pages 60 to 90. Typically, there's a key turning point or reversal halfway between each of those acts, too.

- The stakes get higher as the story progresses. The protagonist makes a crucial decision around page 30 which sets him off on his adventure. Shit hits the fan between 30 and 60, at which point, the protagonist realizes what he must do to resolve the situation. But then shit gets even worse, and the hero's in serious peril (physically, emotionally, or what have you). The final ~30 pages represent his final push against the antagonist(s), with a climax occurring around page 75 to 80, and a denuement from 80 to 90.

- Each scene must move the story forward in some meaningful way. This doesn't have to be overt or action-based; even a dialogue between two characters should create and then resolve something. This can be very subtle, and it's hard to notice in films. But the difference between a scene that moves, and a scene that drags or is pointless, can become readily apparent when reading a screenplay.

- Subtext: characters have personalities, goals, motivations, agendas, desires, and intentions. They don't overtly state these goals or intentions in dialogue. Two hallmarks of bad dialogue are 1) expository dialogue, i.e., dialogue that serves little purpose but to explain things; and 2) dialogue in which characters broadcast their intentions. Characters speak in their own voices. In bad dialogue, all the characters sound the exact same. In good dialogue, the characters have different ways of speaking, seeing things, etc.

- Good plots involve interesting characters that make interesting choices. The plot stems from the actions, or reactions, of a character to his circumstances. Things don't just "happen" to the character. He makes choices, and what happens next is a consequence of those choices.

- As a corollary to the above: interesting things happen when good characters make bad decisions. Think of Frodo and the ring of power. It would have been a pretty boring book (and movie) if he'd just kept the damned thing in his pouch the whole time and never put it on, right? In fact, I'd recommend Peter Jackson's first of the three LOTR movies to anyone wanting a crash course in screen plotting. It's a long movie, and it does slow down from time to time, but it's amazingly structured. Nearly every scene flows logically from the next, and in a subtle way. Every scene is the result of someone's decision, and the tension escalates accordingly.

- The Golden Rule: show, don't tell. Don't use expository dialogue as a substitute for action (and, by "action," I mean characters making interesting choices and doing interesting things. "Action" does not necessarily mean fight scenes or car chases). Don't use narration if you can avoid it. Narration is lazy. Sometimes people have broken the narration rule to great effect (i.e., when you have an unreliable narrator, or a narrator who provides humorous color commentary) -- but 9 times out of 10, narration could have been better substituted by story.