According to Google, John Williams wrote the music to Harry Potter, but it also said that the orchestrator was Conrad Pope Eddie Karam.
Does that mean that John Williams originally did not compose it for orchestra, but rather, some other medium, and somebody else arranged it for that particular ensemble?
Typically a busy film composer like John Williams will write a condensed score, also known as a short score. This will have at least two or three systems and contain all the important musical ideas, and probably a general idea of the orchestration. This will often be hand-written. He'll pass this to his orchestrators who then produce the full score.
