Brown, as a professional author, writes The Da Vinci Code in the same fashion as some other writers accused (and probably rightly so) of supermarket paperback writing: cardboard characters and little frills when it comes the actual narrarition of the book.
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That said, I really liked Da Vinci Code because I found myself interested in the subject matter, and I have always been a sucker for a good conspiracy theory. The characters and even the story itself felt like they were there merely to play out the theory that Dan Brown was presenting