Dud Sea Scrawls The Writing, clearly, is not on the wall!

A screenplay is the most basic foundation of any movie. It is a popular saying that you can make a lousy movie from a great script, but you can never make a good movie from a lousy script.
That said and done, making a movie is a lot lot tougher than writing a script.
The trick here, as it is for every creation, is the ability to either be supremely uncompromising on the script, or have the even tougher ability to be able to step back in the middle of creation and see if it is coherent. This involves, first and foremost, the ability to NOT fall in love with your own work - but to be able to always see it as a layman when you step back to review it. The biggest confusion can arise when one falls in love with one’s own idea and fails to see how it is not coherent as a whole. For, a creation is more than its bits and pieces. When you watch a movie, it is sometimes so easy to tell which scenes have been put in later, and are not congruent with the rest of the script.
To prevent this from happening, hollywood has deployed the technique of getting a script written by one person, and then any changes are done by other good script-writers, who can see it from a fresh perspective, and thus can put in the right touches to make it more complete.
The mark of a supreme talent in direction/script-writing is the ability to have a whole lot of separate scenes, which still are fully consistent with the central ethos of the overall story.