Screenplays are either written on assignment or are speculative depending on how they are commissioned. Many established screenplay writers work on assignment basis.
Assignments for writing screenplays are commissioned by production studios on the basis of the story idea provided by producers or books the studio has purchased the film . The assignment may be for an original screenplay or a screenplay based on a novel, short story, comic book, real life incident or a video game. Sometimes, the assignment may be for rewriting the script , so that it is approved by the producer.
Speculative (or “spec”) scripts are written independently by screenplay writers, so that they can sell it to studios after it is ready. Some writers get a better price for a spec script than a script written on assignment . The writers of spec scripts also have more creative freedom. Writer directors who are making a low budget film may also write a spec script.