"JOHN enters the room. He is Frank's best friend and the life of the party."
OK. We, the readers, are suppose to know this because the writer said it? What makes matters worse is when, right after making this statement, the character of John engages in an activity or line of dialogue not even remotely associated with his "life of the party" description. In a movie script, the writer should introduce every action and/or character the same way the movie goer will be introduced to them on the screen. Now compare the above introduction of John to this one:
"As Frank makes his way through the crowd of people at the house party, his attention turns to where the loudest commotion is coming from. As he enters into the living room we see his friend JOHN surrounded by hoops and hollers, dancing on a table while a beautiful topless blond sits on a chair receiving a lap dance from him."
Get it? Now doesn't this get the point across visually that John is not an introvert? In screenwriting you must always check to see if what you are trying to get across is being transmitted visually at all times. Not to downplay or trivialize dialogue, but no matter if the movie you are watching is good or bad, you can watch a movie on cable, turn down the volume, watch the movie from beginning to end with no sound, and know exactly what the movie is about. Why? Because a screenplay has to be VISUAL or else the movie will not be made. To show you just how important visual writing is, some writers do not write a line of dialogue until the structure and flow of the action is completed from beginning to end. They write the dialogue last because when they go to each scene, know what the scene is for, and what will happen in the next scene, they will be able to write in context.
How NOT to write a screenplay: 101 Common Mistakes Most Screenwriters Make , by Denny Martin Flinn, is probably the first book you need to read if you want to know not only how a good screenplay is NOT written, but also the mindset of the Hollywood script reader. This is important because if you don't impress the script reader, the person the people with the money use to filter out the good from the bad screenplays, no one else will see it. You may not write a great script after reading and applying the wisdom within its pages, but you will not write a bad one.
On the flip side if you want to know HOW to write a screenplay, look no further than Screenplay: The Foundations of Screenwriting, by Syd Feld. I personally recommend this book because not only is Mr. Feld known as "the father of structure," but he deals specifically with the art of screenwriting and less with the business of the movie industry. No need to rush..that part will come later. The Screenwriter's Workbook is another book by Syd Feld and the companion book to Screenplay: The Foundations of Screenwriting.
500 Ways To Beat The Hollywood Script Reader: Writing the Screenplay the Reader Will Recommend by Jennifer M. Lerch concentrates on how to make your screenplay enjoyable to the Hollywood reader and literally gives you 500 ways to avoid having your script scrapped by the reader and passed on to the people who can bring your idea to life.
The Screenwriter Within: How to Turn the Movie in Your Head Into a Salable Screenplay by D. B. Gilles and How To Write A Movie in 21 Days: The Inner Movie Method by Viki King are two other books I personally recommend. Both of these books deal strictly with the screenplay itself and less with the movie industry. Trust me, you will have plenty of time to deal with that hurdle once your final draft is completed.