Some months ago, I interviewed a noted American director who had adapted a book by a noted American author. The director was in a hotel suite. He had a retinue of handlers and his own PA. The author was placed in the corridor outside. He had a wonky trestle table piled with his own paperbacks. While the reporters were lined up to speak with the director, the publicist would attempt to interest them in the author. “He’s signing copies of his book,” she explained. “He’s signing them for free.” The author stared up at the reporters with a mortified smile and said: “Can I go and get some lunch now, please?”

At the time, this sad little vignette struck me as the perfect illustration of the author’s position within the movie industry: stuck on the sidelines, diligently ignored. It could have been a scene from one of F Scott Fitzgerald’s Pat Hobby stories, or an extrapolation of the old joke about the starlet so dumb that she slept with the writer. Except that now it transpires that the joke was on me, still clinging to outmoded ideas, oblivious to changing times. Because the author, it turns out, is cinema’s new auteur. The Hollywood Reporter hails them as “the real superheroes of the film industry right now”.

Fifty Shades Darker: the trailer.

This is not to suggest that every underpaid hack is a Superman now. Instead, the magazine is referring to those bestselling authors who have the clout to ensure their novels aren’t manhandled or damaged on the way to the screen. Specifically, it is referring to the likes of Stephen King and George RR Martin; to Gillian Flynn, who adapted Gone Girl from her own book, and the mighty JK Rowling, who effectively doubles as a de-facto boss at Warner Bros. No doubt it is referring to EL James too.

Friday sees the release of Fifty Shades Darker, the sequel to Fifty Shades of Grey. It’s an adaptation made with the complete support and involvement of its author EL James (AKA Erika Mitchell) who was granted unprecedented control over the production in a landmark deal with Universal. James has final say over everything from casting to script to wardrobe choices. She commissioned her husband (Niall Leonard) to write the script for latest movie. Good news if you’re a fan of James’s steamy source material, in which “desire pools dark and deadly” in every character’s groin and eyes are “brown, like bourbon, but flat”. Obviously, less so if you aren’t.

“Sometimes a novelist’s input can work and sometimes it backfires,” says literary agent Julian Friedman. “JK Rowling might be the most extreme example, in that she’s hands-on in everything, and yet I have the sense that she also listens and takes advice; that she’s a rational person. I thought Gillian Flynn did a pretty good script for Gone Girl. But EL James, I don’t know. I personally thought the first film was dreadful.”

EL James … ‘Good for Erika,’ says Patrick Marber on having his script rejected. Photograph: Pablo Cuadra/WireImage

Friedmann has been an agent for 40 years; he can put the recent shift in historical context. Specifically, he blames the scrapping of the net book agreement, back in the 1990s. This laid the ground for a new business model in which publishers discounted big titles and supermarkets then sold them. It resulted in the rise of a premiere league of bestselling authors who are also brand-name superstars. It used to be that, when adapting a book, Hollywood paid the author to keep well away. Now, all at once, they were forced to include them; to give them cast and director approval; to pander to their internet fanbase by sticking like limpets to the original text. This accounts for the rise of Rowling and Flynn, Suzanne Collins and Stephenie Meyer. Possibly it also accounts for the stiff, reverent films that emerge from the process.

“Generally, I don’t see it as a positive development,” says Friedmann. “The majority of authors are terrified of upsetting their fans and damaging their brand. They become so afraid that they start interfering creatively when they don’t understand the business, the process, the audience. Film communicates in a different way to books. If the point of an adaptation is to open the work up to a new audience, then I’d say to an author: ‘If you upset some of your existing audience, that’s OK. You’ll keep most of them and you’ll add others.’”

Failing that, at least have the confidence to defer to others from time to time. The author David Nicholls, who has adapted several of his own books for the screen, likens the process to cutting your own hair: “Sometimes you want someone to take a look and make sure the back looks OK.”

Last year I interviewed the playwright Patrick Marber, who had been hired to rewrite the Fifty Shades of Grey script. Marber explained that at the time the original star (Charlie Hunnam) had just walked and the production was in crisis. He took a radical approach to the existing work. He stripped the drama down to a straight two-hander and exuberantly rewrote the bulk of the dialogue. The director (Sam Taylor-Johnson) was thrilled; Universal reportedly was too. Yet it was the author who had final say. “And Erika hated it,” Marber recalls now with a shrug.

At the time, Marber was upset. He thought his work had been good; he was sorry to see it dismissed. Since then, however, he has come to view things differently. “My attitude is actually: ‘Good for Erika,’” he says. “She wrote the damn books. She cut herself a great deal. The first film was a huge success and she has complete control of the second one too. I happen to disagree with her about my script. But I still champion her right to not like it and to throw it away.”

Marber can be forgiven for nursing a grudging admiration for a fellow writer made good. Here was a historical wrong being righted; the soulful, lowly author being brought in from the cold. But it leaves me pining for the good old days of creative vandalism, of fresh pairs of hands and for adaptations that bound off in unexpected directions. Because if the novelist is the parent to the book, then the final measure of good parenting is the ability to let go. Better that, surely, than hot-housing the thing all the way through its life, like a paranoid tiger-mom administering to some grotesque, pampered adult.

And besides – unlike a child – the actual book never leaves. The work is the work; the film-maker can’t touch it. Today’s bestselling authors might take a leaf from the book of James M Cain, author of Double Indemnity and The Postman Always Rings Twice. “People ask me: ‘Don’t you care what they’ve done to your book?’” Cain once remarked. “I tell them: ‘They haven’t done anything. It’s right there on the shelf.’”

Fifty Shades Darker is released in the UK and US on 10 February