Orson Welles once said editors, not directors, were the real makers of films: the notion of directing a film "is the invention of critics", he insisted.
No self-respecting writer would let that pass unchallenged, but the people who really get the films made are the producers - directors are, to a greater or lesser extent, at their mercy.
Successful documentary producer Simon Chinn won't buy the idea that he runs the show. Producer of the Oscar-winning Man on Wire, Project Nim, sleeper hit Searching for Sugar Man and the astonishing The Imposter, which opens here today, Chinn describes producing as "something of a black art".
"You have to adapt yourself to the project," he says, speaking from his London home. "James Marsh, with whom I made Man on Wire and Project Nim, is an incredibly able film-maker who nevertheless appreciates interventions when they are timely.
"By contrast, Malik Bendjelloul, on Searching for Sugar Man, wanted a very intense dialogue. He was very much on his own before he came to me and he was desperate for a real, intensive, creative exchange which had been missing for him."
The impostor of The Imposter (the film's title adopts the American spelling) is Frenchman Frederic Bourdin, nicknamed The Chameleon by the tabloid press, who has assumed dozens (he claims 500) false identities since childhood, including, on three occasions, those of missing teenagers.
The film concentrates on the 1997 case in which Bourdin took the identity of Nicholas Barclay, who had gone missing three years earlier from his home in San Antonio, Texas, at the age of 13. "Discovered" cowering in a phone box in Spain, Barclay/Bourdin was reunited with his "family", who accepted him as their own, even though he had a French accent and his eyes were the wrong colour.
A compelling, creepy and quasi-farcical study of both deceptiveness and credulity, it is an almost eye-poppingly improbable story. And its impact is considerably enhanced by a visual style and sound design that endows supposed re-enactments with a chilling ambiguity: the boundary between the real and the enacted Bourdin constantly blurs. Chinn says that style was director Bart Layton's.
"On every film the look and the style evolve. Bart had a bold vision for the film at the outset, and he had developed the idea of having actors lip-synch to real-life audio in a television series called Banged Up Abroad.
"They're not really re-enactments anyway, but an interpretation of slippery truth: they're supposed to be unreal, that's the point."
Chinn is at the top of the non-fiction film-making tree in an age when it is going from strength to strength. He says theatrical documentaries have grown in popularity.
"Television doesn't offer film-makers the budgets and broadcasters want documentaries to be far more populist and mainstream and formatted.
"So there has been a growth in the market for commercially viable but adventurous documentaries, and audiences have come to see that documentaries can play like the best fiction films - if not better."
He makes the point that the most successful documentaries in recent years - Touching the Void, Man on Wire, When We Were Kings - are first and foremost narrative films.
"They tell a really good story. The fact that they are documentaries is neither here nor there. People speak of documentary as a genre but it is really a form: the genre is narrative."
Chinn's biggest success was Man on Wire, about Philippe Petit, the Frenchman who walked a tightrope between the twin towers of the World Trade Centre before they were even completed. He says it took him months to negotiate the rights.
"Several people had tried to make the documentary and for whatever reason the proposal broke down - or maybe the film-makers realised the challenges of working with Petit."
He is referring to the man's legendary protectiveness of his story; Petit told TimeOut he thought the film should have been about "the sky and the seagull and the dialogue of me and the cable".
"There were big battles we had in the cutting room," says Chinn. "But we ended up making the film we wanted to make and he came round to understand the decisions that we had made and respected them."
In the case of The Imposter, when Layton first contacted Chinn and his co-producer, he had "taken a real punt" in shooting an interview with Bourdin.
"He cut it into a promo or taster that was extremely compelling. But he had none of the rest of the access to the family - the mother and sister were adamant that they would not talk, so that was the challenge."
The finished work is one that leaves the viewer almost speechless with incredulity.
"The fact that they are prepared to accept into their life a man who is - at least to us - so obviously not their son," says Chinn, "is utterly confounding for ordinary people."
Who: Simon Chinn
What: The Imposter
When: Opens January 10