Let's deflect that question by asking another one.
How putrid must a stinking glob of gelatinous offal become, before you decide your nostrils have been raped?
|A still from Basket Case (1982)|
Fruit ripens, and rots soon after. In a way you could see rot as a surfeit of ripeness, as the natural sugars reach a climax, so to speak, and begin to ferment. (At this stage, they become particularly interesting to moose, but that’s neither here nor there.)
A bad movie is a cultural artifact that went straight from green to rotten. It never passed through an edible stage.
At times it’s hard to tell good from bad cinema because, let’s be honest, movies aren’t food and you can grow on a terrible diet. Most people will never know the difference between Inception and Eyes Wide Shut.  Once, a PhD student reacted to my mention of Ingmar Bergman with “Bergman is boring!” (That was her appraisal of someone Woody Allen calls the greatest filmmaker in History.)
But that doesn’t mean it’s all subjective. Some movies are indeed better than others -- this is art, not democracy.
A bona fide crap movie must present at least 4 of the traits below:
a) no-name/no-talent actors
b) ludicrous premise
c) terrible script
d) pretense, which includes
i) plagiarizing older movies
ii) recycling old crap believing this is somehow ironic
iii) trying to overwhelm you, the viewer, with ‘meaningful’ ideas
iv) poor attempts at elevated speech
e) a staggering lack of originality
f) a staggering excess of originality
h) gratuitous sex and violence
i) laughable special effects
Shock factor alone doesn't mean a movie is bad. Take Cronenberg's coldly, subtly violent Spider, the film that showed me Ralph Fiennes could act, and how. It is the story of a man who misremembers his childhood. Spider's alcoholic father killed his wife and brought a whore to the conjugal bed. Years later, Spider still dwells on his past. Stuck in a halfway house for the mentally unstable, he plots to get out; things take a nasty turn. (I won't spoil the movie for you.)
Spider serves up a fetid broth of bodily fluids, spousal abuse and crushing loneliness, and yet all the dirty strands come together in a delicate web of meaning. All that you see obeys the principle of relevance.
You know what they say about ideas and execution? It’s execution that counts. Jacques Tourneur’s Cat People (1942), shot on a $134,000 budget , was nevertheless a worthy exercise in the use of shadow and suspense. Cat People doesn’t show you anything you don’t need to see, and obscures much of what you’d want to see.
Ingmar Bergman's Persona  freezes you to the bone, turns you inside out. It relies on camera work, refined dialogue and subtle, ultra-professional acting (the kind that makes you forget you're watching a performance) and -- most importantly -- the viewer's intelligence and sensitivity. Common to all bad movies is that they don't ask you to think, only that you submit to them.
Elegance is knowing when to leave things out; bad movies thrive on excess. They say yes to everything.
Like, can we have katanas popping out of girls’ butts?
Can we have machine-gun nipples?
Can we have giant robots smashing buildings that bleed?
I am obviously – OBVIOUSLY – because everybody’s seen it, right? – referring to Noboru Iguchi’s masterpiece, RoboGeisha. RG clearly suffers from f), a staggering excess of originality. Geishas turning into mini-tanks? Assassins shooting corrosive milk out of their nipples? Come on.
This is what happens when you don't kill your darlings. Must every story become an orgy of blood and mayhem? Maybe we should stick to this principle: You can include anything you want in your story, but you can't include everything.
So your budget is unlimited, your imagination -- inexhaustible. It makes no difference whether Krovelzaxx of Kroth ruled ten, or ten thousand planets; or whether Juarez Merkel packed one or two pantropic ablators. As a writer, your currency is time, not dollars. But at some point your story begins to rot. All that sweet, sweet plot juice reaches a climax, you know? And then it begins to ferment.
Because this subject is so huge, there's a part two now.
 The difference being that Inception is a pretentious, horribly self-important movie, whose premise was lifted wholesale from a Donald Duck comic. Believe it or not.
 $1,772,442 adjusted for inflation. To put things into perspective, 1408, starring John Cusack and Samuel L. Jackson, cost approximately 25 million; the movie takes place mostly in a hotel room, with few actors, and employs special effects rather discreetly. An utter train wreck of a horror movie, Pumpkinhead, cost $3,5M to make. And yet, it shows you a lot more than you'd like.
TERRIBLE MOVIES, IN NO PARTICULAR ORDER
Anything with Adam Sandler in it
The Gingerdead Man
Anything with Rob Schneider in it
The Passion of the Christ
Anything with Jack Black in it
Transformers I, II, III, IV through LXXVI
You've probably noticed that I included a handful of blockbusters. I don't want to give you the impression that I look down my nose at popular fare, or campaign for creatively bankrupt pieces like Sleep. There's more to life and cinema than the art-house sublime, but film is a human endeavor and an art form. It must provide a vehicle for individuals and their vision; not put on offensively vacant clown shows. That vacancy, that emptiness has so corrupted the art of film, it's no wonder more and more people shun the box office.
Time and again, Hollywood has proven that a huge budget can't buy you taste or sensitivity.
By the way, if you use your phone in a movie theater, somebody should put fire ants in your underpants.
What can they teach you about writing? -- is a weekly series of articles drawing on public statements by talented people, and how such statements apply to the act of writing. “Talented people” does not mean they’re entertainers, nor do I expect you to agree with my definition of talent at all times. In early 2012, I decided to expand the scope of these articles to include remarkable characters in works of fiction.Read more in this series.