When Cairo and Brigid O'Shaughnessy meet, they make references that the reader and Spade don't initially comprehend.
What is the meaning of the reader in this sentence ?
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Although your source is a review of the film based upon Dashiell Hammett's novel The Maltese Falcon, for some reason the reviewer, in this sentence, writes as if the novel, and not the film, is the subject of the review. The reader in this sentence refers to the person who is reading the novel. The reveiwer should have written the audience instead.
Note that reviews at IMDB.com are written without any editorial supervision. Anyone at all can write and publish a review there, and their work should not necessarliy be taken as exemplars of proper grammar or usage.