Despite the all-caps nature of his email, this is a brilliant piece of advice that applies not only to screen writers, but novel writers as well.
For example:
ANY SCENE, THUS, WHICH DOES NOT BOTH ADVANCE THE PLOT, AND STANDALONE (THAT IS, DRAMATICALLY, BY ITSELF, ON ITS OWN MERITS) IS EITHER SUPERFLUOUS, OR INCORRECTLY WRITTEN.
and:
START, EVERY TIME, WITH THIS INVIOLABLE RULE: THE SCENE MUST BE DRAMATIC. it 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.
and:
I CLOSE WITH THE ONE THOUGHT: LOOK AT THE SCENE AND ASK YOURSELF "IS IT DRAMATIC? IS IT ESSENTIAL? DOES IT ADVANCE THE PLOT?
ANSWER TRUTHFULLY.
IF THE ANSWER IS "NO" WRITE IT AGAIN OR THROW IT OUT.
Seriously, just read it all. It's brilliant.