I was talking with my writing partner last night about what makes a script a fun, easy read. Having a good writing style (prose/dialogue/layout) helps greatly, of course, but equally crucial is the way in which the story unfolds. In the course of our discussion, I realized that two principles in particular are key:
1. Almost any reversal/twist that hurts the protagonist will be welcomed by the reader/audience. They want to see how the protagonist is going to get out of whatever fresh hell this is, so they'll keep reading/watching.
2. However, a reversal/twist that helps the protagonist will usually be met with something between wariness and outright scorn -- unless the twist has been set up beforehand, in which case it can be very satisfying.
It might seem odd that I use the terms "reader" and "audience" interchangeably, given that a person reading a script most likely knows a fair amount about screenwriting, whereas the average moviegoer has never so much as cracked a book on the subject. In fact, audiences know quite a bit about writing, even if they don't know they know it. They certainly know enough to distinguish clever writing from lazy, sloppy writing.
Case in point: If James Bond spends ten minutes/pages battling his way through a small army of Russian soldiers to retrieve a jetpack the size of a pencil, the audience will have no problem believing that he can use that tiny gadget to fly to safety. On the other hand, if Bond is being chased on foot by evil agents through the streets of London and just happens to come upon an unlocked car with the keys in the ignition, the audience will call bullshit immediately. And why? I mean, which is actually more likely -- a six-ounce cylinder that makes you fly, or one person in a huge city accidentally leaving their keys in the car?
The answer is that it doesn't matter. The audience isn't responding to the actual plausibility of those things; they're responding to the strength (or weakness) of the writing. When they say, "That wasn't believable!" what they really mean is, "The writer didn't earn my suspension of disbelief."