I have posted this many times before - look at the artwork. For example: If you did not know that the convention in Leonardo's time was to depict St. John as looking rather feminine, you would think that "The Last Supper" showed a woman sitting next to Jesus.
Again: the DESCRIPTION is accurate, the INTERPRETATION is fiction.
Actually, if you read Dan Brown's own web site, those are Dan's real interpretation. Brown believes it to be the truth. He clearly says he disagrees with the scholars who debunk his book.