Robin Williams, wrote Kenneth Turan in the Los Angeles Times last January, has two interview modes: wildly funny and thoughtful. Put him on a stage, though, and ‘thoughtful’ usually gets put on hold. So it was more or less inevitable that, when he stood up after the much-applauded screening of One Hour Photo at Sundance, Williams should start with a joke. “I did this movie,” he said, “because Mister Rogers on Ice was already taken.”

It provided the evening with its first laugh: One Hour Photo is definitely at the thought-provoking end of the spectrum. Seymour Parrish, the character Williams plays in music-video director Mark Romanek’s debut feature (he has previously worked with everyone from Michael Jackson to Nine Inch Nails), is no laughing matter.

“People always say, ‘Oh, you play such nice people’, says Williams. “This man is nice, but with a dark side. It’s been exciting to play that. He does things that are creepy and bizarre. It’s interesting stuff to inhabit a real and very, very fascinating character.”

“Everyone has experienced Sy moments - the inability to connect with other people, of always being a little off, that kind of loneliness,” notes producer Pam Koffler. “The trick to Sy is making him sympathetic, without making him saccharin - and that’s something that Robin’s really managed.”

In point of fact, Williams’ last three roles - in this movie, Death to Smoochy and Insomnia - have all been a long way from Patch Adams. Seymour - known to everyone as Sy - may be odd, in other words, but he’s not that hard to identify with. A loner with a tenuous grasp on reality, he tips dangerously close to psychotic behaviour when the one thing that keeps him grounded falls apart.

That thing is family. But it’s not his own family (he has none): it’s the Yorkins, who appear to have everything. Nina (Connie Nielsen from Gladiator) is beautiful, calm and loving; Will (Michael Vartan, the sympathetic teacher in Never Been Kissed) is handsome and dynamic; nine-year-old Jake (newcomer Dylan Smith) is just starting out on the school football team. “This family seemingly has everything together,” jokes Vartan. “Even the dog matches the furniture.”

writer/director Mark Romanek.

Sy has known them, watched them grow, since Jake was a baby. But he has never really met them: as the title of the film suggests, Sy works behind the photo counter at the local SavMart. His knowledge of the Yorkins comes from having handled almost a decade of family snaps - copies of which cover the wall of his sparsely furnished apartment. We’re talking major-league obsession here.

But Sy is not really a voyeur - he doesn’t want to pry on the Yorkins’ intimate moments or become directly involved in their lives in any way. He’s more of an emotional parasite who needs this perfect family’s happiness in order to give some meaning to his own life. But, of course, the Yorkins are no more perfect than any family who live in the real world which Sy can only see through the photos he develops.

“What struck me,” notes Nielsen, “was that feeling when you’re 15 years old and you think, ‘If I could just be that person, or have that money or whatever, then my life would be perfect’. Then you grow up and realise that there is no such thing as perfect. That is one of the things Sy needs to realise.”

“They’re not like a Norman Rockwell family,” adds Williams, “but almost like families you see in advertisements now: they’re young, beautiful and perfect in terms of that ideal image, and they seem to have everything. And that’s what Sy finds fascinating because he is totally the opposite; not attractive, he lives alone. They are like the exact counter-pole to the universe for him. And he’s drawn to that.”

What destroys this cosy little dream-world for Sy is two things: first, he gets a set of photos from another customer which seem to suggest that Will is having an affair; and, when his boss runs a check and finds the discrepancy between the number of photos printed and the number actually sold (explained, of course, by his private Yorkin Family collection), Sy gets the sack. All the things that gave meaning to his life are removed and there is nothing for him to replace them.