Something like 1 out of every 1000 scripts are actually bought. Most are average. The problem is what happens when all the studio execs put their two cents in to make a script what they consider marketable. It had the rare advantage of actors and a director that knew what they were doing. Changes are made because of actor ego conflicts, location difficulties and many other problems.
Skilled actors know how to make dialogue sound more real.
They can make changes in the script to improve it that the writer never could have foreseen. So much is left up to the chemistry of the cast and crew assembled that the writer sometimes feels like a footnote in his own creation by the end of production. And this is the way the studios prefer it. They want an idea that sells more than they want good writing. Not to say that they would really have it any other way.