Multiple Writers on a Screenplay...
I often see movies that have more than one writer - how does that work?
This is a really good question and like most things in the movie business the answer is really situational, dependent on why those writers were convened to work on the script. These are some of the examples.
1) A producer (money person) has a story idea - but she/he is not a writer. So one writer or several are hired to create the screenplay from the original idea. In the film credits this usually is phrased by STORY BY: xyz person SCREENPLAY BY: abc person
2) A writer (you or me) creates a 'spec' or 'indie' script -- a script that has no talent (producer / director/ name actor) legally attached. The script is written with the idea that the writer will eventually sell it. A producer or director reads the script and really likes it except for.... whatever reason. So they contract with the writer (through an agent or lawyer) to buy/or option the script if the writer will make the proposed changes. The writer agrees on the proposed changes are useful, and makes the changes, and the producer then buys the resulting script.
In the process of trying to find the perfect director or name actor (talent) to attach to the script - more changes are suggested. The writer decides these changes will really compromise his original idea/story/ theme. For her own reasons the writer refuses to make those changes. The producer now owns the script. So he /she contracts with a different writer to make the changes the 'talent' wants but the creating writer balked at doing.
3) On the road to finding the perfect blend of producer/ director/ actors - the script goes through many rewrites. Ideally the creating writer makes the changes. But sometimes another writer who has an established expertise - such as dialogue will be called in to freshen the dialogue. Or they might want to skew the dialogue to fit a particular actor's talents (think Schwarzenegger’s - 'I'll be b-a-c-k....'). Often these collaborations are quite amiable, with each writer understanding they bring to the table their own particular strengths.
There are many, many other situations where a whole stable of writers take turns polishing a script for various complex reasons. Filmmaking is a COLLABORATIVE ART and your original screenplay often is only a springboard for other creative talents that will bring your script to the big screen. You really need to be able to stand back and let that happen. And never sign a deal or contract without the advice of a really good agent or film lawyer who will protect your rights in the event that you encounter of any of the above creative writing situations.
I hope this helps. And I do wish you good luck with your screenplay.