“We talked about the character and that he views himself not as an evil person, just as a man who is righteous in his own way,” says the star. “It’s almost like he is doing good, in a bizarre way.

“When I first read it,” he continues, “I thought it had so many interesting turns. You think it’s going one way, then it gets very disturbing… disturbing in a good way - if you can be disturbed in a good way! His focus changes from one scene to the next and I found myself not knowing what was going to happen, which is good. I found myself drawn into the character almost to the level of the thought process. That’s what fascinated me.”

Offering Williams the part of Sy was initially a long shot on the part of Romanek and producers Christine Vachon and Pam Koffler (Boys Don’t Cry). An independent movie with a first-time director, it was not the kind of thing that usually showed up on Williams’ radar. Even more unlikely, it had arrived at Vachon and Koffler’s New York production company, Killer Films, more or less unsolicited.

But the pair were impressed. “The script was intriguing enough for us to feel like we wanted to sit down and talk to Mark,” remembers Vachon. “That’s part of the process. Will a director convince us that a script we felt was good is even better? Do they make us excited about the journey from the page to the screen?”

Robin Williams as obsessive photo-counter guy Sy

Evidently he did: impressed by Romanek’s evocation of the classic ‘lonely man’ movies of the seventies - films like Taxi Driver and The Conversation, which broke the rule that a central character has to be likeable for audiences to engage with him - Vachon and Koffler decided to take the film on board.

They also figured that an identifiable star in the role of Sy would lift One Hour Photo onto another plane. They wanted, says Koffler, “someone with that kind of charisma who was willing to play a character unlike anything they’d ever played before”. Romanek suggested Williams and, after initial disbelief (evidently shared by Sundance audiences before they had seen the film), the two producers saw the logic behind the suggestion.

“I think audiences will be excited by the idea of seeing Robin do something that they haven’t seen him do before, or not in a long time,” says Vachon. “Robin has done dramatic roles all through his career but they certainly aren’t Sy.”

Connie Nielsen, Dylan Smith and Michael Vartan as the Yorkins

The next hurdle was Williams himself. But, when the actor agreed to have lunch with Romanek, the latter didn’t realise that he had already decided he wanted to play the part. The young writer/director, reckoning that the meeting was just a polite way for Williams to say no, was initially so surprised when he heard a different answer that, recalls Williams, it was Romanek who said “No!’

Accepting that, having turned 50, his days of playing an eternal adolescent may be over, Williams explained the decision by telling the Sundance audience he needed to look for different and challenging roles - but not go too far. “Going too much one way,” he told Turan, “is not good. Goebbels: The Musical - that I don’t want to do.”

But he did want to enter into the slightly unbalanced world of Sy Parrish - a world which is centred around the blandly antiseptic supermarket where he works. “The SavMart store is like hyper reality,” says Williams. “It’s like a lot of those big market-type stores with surreal abundance and, amongst that, he sits in this little corner. Everything is this kind of white, bright light all around and Sy could all of a sudden just disappear in his blandness. In the outside world he stands out, especially when you get near the Yorkins’ house, which is very warm and incredibly beautiful - almost painfully beautiful because it is his idealised home.

“In his own life, you know, things are very much in place. It’s very distinct: this is not someone who acts out. Everything is planned - except for when things start to fall apart. And when his world falls apart, it falls apart big.”