In the past few days I've waded through dozens of reviews, nearly all negative, of the film of The DaVinci Code, but none of the comments I’ve read offer much insight into what went wrong with the adaptation of such a sensationalist pot-boiler. I’ve written at length elsewhere (for a UK writers’ magazine) about the related book-film adaptation issues, and about how the book came from a background of development of the ‘film-friendly novel’. Anyone interested in this can read my feature here: The Da Vinci Formula.
My own guess, before having seen the film, is this. The novel was too long (at 593 pages) and wordy to work within the feature-film screen-time limitations (here stretched to its limit at 150 minutes), and within the Hollywood mainstream film’s conservative conventions – and the usual studio requirements of broad demographic appeal. (Sony actually toned down the music score on the British film censor's advice it was too scary, in order to get it a more general ‘12A’ release certificate in the UK). The church-led campaigns that began attacking the project pre-release may well have stifled creativity and sensible decision-making. The industry trade paper Variety classed it as ''oppressively talky.''
For some of the audience who hadn’t read the book, it may have been a a case of 'TMI' - too much information both as to quantity and to its ‘revelatory’ nature, which can create a certain culture shock and incredulity among many raised as traditional churchgoers.However this would not necessarily harm ticket sales due to the film being an “event” and a conservation piece, and any inability to follow the film’s dense yet obscure plot might only encourage viewers to read, or re-read, the book for clarification.
One could also argue the book actually used the sleight-of-hand technique of overloading readers with so much 'revelatory' information they lose their critical focus, and don't notice the plot clues don't really add up. In the film however, the necessary pruning away of background historical explanation leaves the plot leaps more exposed, with the explanations having to be squeezed into and around the obligatory Hollywood-thriller chase and 'suspense' scenes.