One of the most annoying things that not-so-good playwrights and screenwriters do is create characters/scenarios that utterly lack integrity.
A producer/director in the local independent movie scene recently made a movie set in "space" somewhere, and although I think the plot is lame and the recycling of Star Wars technology even lamer (the sexy ladybot has to use a light saber to fight people wielding gigantic space blasters) the worst aspect is the script going for whatever is narratively convenient, rather than letting the characters and situations play out organically.
Most egregiously handled is the dumb-guy character, who is dressed like a cowboy and speaks with a Southwestern accent. Right in the middle of the movie, he takes time off from drooling and bumbling around to solve the movie's big mystery - he figures out that the hero of the story has returned incognito to seek vengeance. He lays it all down like a perfect logician: "so whether or not he is Mr. X, he's out to fry everybody connected with Mr. X's death."
You know the screenplay is bad when the dumb guy is given the job of solving the mystery.
The best part of this mystery-solving is that right before the dumb guy solves the mystery he is called "nitwit" and "idiot" - but that's not unusual - in fact, virtually every character in the movie insults the dumb guy's intelligence, except for directly after he solves the mystery, when his boss (a pointlessly long-winded and affected douchebag) calls him a king among men. To which the dumb guy responds something to the effect of - and I am freely translating here - "duhhh.... I'm back to being dumb again now that the mystery is solved."
They actually wrote a character whose job it is to be insulted the entire movie, except when he's solving the mystery.
But you know what - who cares? It's a "genre" movie right? As long as there's lasers and mutants and ladybots in tight-fitting outfits and actresses who get nekkid for free for the director, it doesn't matter how stupid the screenplay is. They'll just keep cranking them out, and there are people in Japan, apparently, who will watch them.
As George Lucas, with his post Harrison Ford Star Wars films, and Steven Spielberg with his last Indiana Jones film (proving that Harrison Ford can't save every movie) demonstrated - it doesn't matter how good the director is, or how big the budget is, if the screenplay is bad, the movie is bad. Period.
So of course screenwriters are the lowest of the low in the movie world's pecking order and screenplays are cobbled together as an afterthought. I guess space-western director guy figures if the big boys make movies with shit scripts, why can't he?