I don't know if "Harry Potter and the Order of the Phoenix" is a good movie — I haven't seen it. But I'm pretty certain that it shouldn't be judged as a movie at all. It is a visual representation of a book in which millions of people are already heavily, even cultishly, invested. When we see it, we are taking part in a ritual that addresses the old, and to me irrelevant, question of whether a film is "faithful" to its source — in this case, the fifth novel in J.K. Rowling's series about a brave and resourceful teen wizard. Of course the picture will be faithful. There would be no point in making it otherwise.

There is something paradoxical in the demand for slavish care in the adaptation of hugely popular novels. The people going to see it this weekend are just out for a rattling good yarn with plenty of special effects, and you probably could put a few variants on the book's plot over on them without eliciting an outcry. But why bother? It is foolish to risk the cult's wrath — or its return business — by offering a product tainted by some screenwriter's nifty new idea.

It's a very lucky novelist who creates a sacred text, something the movie folks dare not screw around with. Leo Tolstoy was not at first fortunate in this regard — the first screen version of "Anna Karenina" permitted her a happy ending. Neither was Herman Melville: There's an adaptation of "Moby-Dick" in which Ahab finds romantic fulfillment instead of a tragic fate in the final reel.

Joseph Campbell, the great analyzer of myth, once defined movies as "the genial imaging of enormous ideas." Even Dashiell Hammett had to endure two ludicrous adaptations of "The Maltese Falcon" before John Huston finally got it right in 1941.

This suggests that when it comes to adaptation, neither literature's high end nor its low end is likely to gather about it an impassioned crowd whose demand for faithfulness weighs heavily with the studios. There are exceptions, of course. Back in the '30s, there were delightful movie versions of such beloved classics as "Little Women" and "David Copperfield," not complete in every detail but engagingly true to the spirit of those novels. And from the low end of the literary spectrum there was, in 1944, Billy Wilder's take on "Double Indemnity," which its author, James M. Cain, considered better than his hastily written novel.

Still, this generalization applies: Movies are a conservative, bourgeois medium, which means that if a weighty cult is going to gather around a novel, in effect asserting its right to final cut of the screen version, the work in question inevitably will be something smack in the middle of the middlebrow range. Case in point: "Gone With the Wind." Its producer-auteur, David O. Selznick, fussed obsessively with that film; his major players had to look the way the book's readers had, as we now say, "pre-visualized" them, and he took the risk of a four-hour running time so he could cram as many of the book's beloved details onto the screen as possible.

I've always thought the result was a crashing bore — the ultimate "woman's picture" masquerading as an epic — but Selznick's potential critics were disarmed, and the audience went along with them. They weren't going to spend the time the movie demanded and then walk out disappointed. Something similar occurred with the two-part adaptation of "The Godfather" (forget that third one) and, I suspect, with the "Potter" series. Sheer length becomes the proof of adaptive integrity — these guys obviously cared enough to send us their careful best — and that expensive effort renders moot the question of whether their time (or ours) was well spent.

Movies, as they have developed over the past century, are a romantic, melodramatic, fantastical and, above all, narrative-driven medium. They remain consistently dependent on the most popular and time-tested storytelling forms — everything from the fairy tale to the mystery story — for their basic materials.

Even when they are creating an original story, the movies remain in thrall to the basic tropes of 19th-century fiction for their most reliable effects. In our postmodernist age, they are stubbornly premodernist in their tastes. They only rarely encounter a fan base as exigent as that of Harry Potter, holding them to the highest standards of faithful replication. In the new digital age, this irony abides: A technologically venturesome medium remains wedded to Dickensian storytelling — and, a few cinematic theorists aside, everyone is content with that state of affairs.

Richard Schickel is a film critic for www.Time.com. He wrote this commentary for the Los Angeles Times.