I would say, A, you can't really teach anyone how to write good characters, because it's something you have to teach yourself or already have innately in you as a storyteller, and, B, coming up with a good screenplay is a very, very small fraction of what it takes to be a good screenwriter.
What happens is, you write a spec, people get it, they see your writing, they see you're good, they bring you into their office, and they say, 'Boy, that spec was really good - we'll never make that in a million years. We have rights to the board game of Monopoly. What do you think about a Monopoly movie?'
There were guys in 'The State' who would take one script and rewrite it and rewrite it and rewrite it and fight for it for a whole season, and after a couple of seasons, you realized that doesn't work. You have to just be willing to throw something away, no matter how good it is, and write a better joke.