From America's greatest living dramatist, David Mamet:
"[Every scene] must start because the hero HAS A PROBLEM, AND IT MUST CULMINATE WITH THE HERO FINDING HIM OR HERSELF EITHER THWARTED OR EDUCATED THAT ANOTHER WAY EXISTS."
You can read Mamet's entire manifesto here.
One other tidbit I loved:
"ANY TIME TWO CHARACTERS ARE TALKING ABOUT A THIRD, THE SCENE IS A CROCK OF SHIT.
ANY TIME ANY CHARACTER IS SAYING TO ANOTHER “AS YOU KNOW”, THAT IS, TELLING ANOTHER CHARACTER WHAT YOU, THE WRITER, NEED THE AUDIENCE TO KNOW, THE SCENE IS A CROCK OF SHIT.
DO NOT WRITE A CROCK OF SHIT.
WRITE A RIPPING THREE, FOUR, SEVEN MINUTE SCENE WHICH MOVES THE STORY ALONG, AND YOU CAN, VERY SOON, BUY A HOUSE IN BEL AIR AND HIRE SOMEONE TO LIVE THERE FOR YOU."
P.S. I always remember the nickname my old friend Alison Dickinson gave to "Glengarry Glen Ross": "The "fuck" movie. Because every other word in the script is "fuck."