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.

A screenplay may be original or may be based on a story or novel. When it is based on a story,  the author may be paid a  substantial amount for the television or movie rights of the story. The screenplay may be written by another person, but the author is often consulted. In the movie credits, the writer is only credited with the story for the movie, if the screenplay is written by someone else.

The format of  a script or screenplay varies , but depends on where it is used. Scripts written for films follow a standard studio format  which stipulates how dialogs, action, characters and other elements should be presented on a page. The scripts for television follow a similar format.