The writing techniques used to develop a short story are probably different from those used in developing a novel. My question is, how should I approach writing a movie script? Given the main storyline of a movie usually could be written in maybe one or two lines, how should my approach be to develop the main idea I have on my mind and turn it into a movie script?
Writing a script for a movie bears little relation to writing either short story or a novel. There are so many reasons why, but to bullet point them:
...and every one of these rules can be broken if you're Aaron Sorkin. It's just different, and it's no less demanding or difficult to do.
There is a book written by author Alexandra Sokoloff that might help. I've not read it but the articles on her blog are well worth a read.