You've just agreed with it in your disputation about Harvey. Now, apply the same reasoning to your belief in God. Neither character's existence is supported by anything in reality. I know Harvey is a fictional character because he was a character in a fictional film. Those two do not necessarily correllate.
Fictional films can certainly include real characters and still be fictional.
The fictional part refers to the story being told and not the individual characters who are parts of the story. That works in reverse, as well. A non-fictional film can include fictional characters. You must separate the story from the characters. I know the film was fictional because it was published as fiction and no claim was ever made it was not.