Screenplay : what seperates it from a Novel ? What is an Adapted and Original screenplay ? What gives 'Screenplay by' and 'written by' Credit ?

Simply put, A screenplay is a script for a film or television show. It includes dialogue as well as character actions and movements.

Is it a guide, or an outline, for a movie ? A blueprint, or a diagram? Or maybe it's series of images , scenes, and sequence strung together  with dialogue and description, like pearls on a strand? Perhaps it's simply the landscape of a dream ?

Well , for one thing, a screenplay is not a Novel, If you look at a novel and try to define its fundamental nature, you'll see that the dramatic action, the story line, usually takes place inside the head of the main character, through his/her point of view. We are privy to the character's thoughts, feelings, emotions, words, actions, memories, dreams, hopes, ambitions, opinions, and more. 

A screenplay is different, Movies are different. Film is a visual medium that dramatizes a basic story line; it deals in pictures, images , bits and pieces of film: the nature of the screenplay deals in pictures, A screenplay is a story told with pictures, in dialogue and description, and placed within the context of dramatic structure. That's what it is; that is its nature. Just like a rock is hard and water is wet.

what's the difference between  An Original and   Adapted screenplays ? Adapted screenplays are based on other works, while original screenplays are based on the writer's own idea. In practice, the distinction is a bit more complicated. Making things even more confusing is the fact that the Adapted category is where all sequels fit inro by default , Toy story 2, 3, 4 , Munna Bhai 2, Hera pheri 2, Krish 2, 3 , etc. Because these sequels are based on characters from original movie or first parts. This distinction between original and adapted works is necessary Because film studios or producers often buy the rights to specific books or magazine articles, and then hire screenwriters to turn them into scripts.

So, a 'Written by' credit goes to a original Screenplay and 'Screenplay by' credit goes to adapted screenplay. In some cases the dialogues are written by a different Writter, often in adaptations to a different language, then another credit of 'Dialogue writter' is also given.

And in no way Adaptation means Plagiarism. Plagiarism is when you don't give due credits and royalties to the original sources ! The distinction between Inspiration and plagiarism is a never ending debate , though legally their are few criteria's for distinction. But, Art and Craft are Subjective entitites, there cannot be a Yardstick to this differentiation!