By Caroline Westbrook BBC News Online entertainment staff

Having spent the last few years committing himself to such saccharine projects as Patch Adams and Bicenentennial Man, it was beginning to look as though Robin Williams would never appear in an interesting film again.

But there have always been two sides to the funnyman's film career.

There is the one which sees him playing the curious manchild (think: Hook, Jumanji, Toys, Flubber and Jack), and the one which shows a more complex dark side, in films like The Fisher King, Good Will Hunting and The World According To Garp.

It is the latter group of films which do a better job of showcasing Williams' acting talents, and this year he has already added to the crop with the recent thriller Insomnia, and the dark comedy Death To Smoochy, yet to be released in the UK.

Sy becomes obsessed with the Yorkin family

Now we have One Hour Photo which gives him one of his biggest acting challenges to date, as seemingly harmless photolab worker Sy Parrish, who hides his delusional fantasy life beneath a creepily bland demeanour.

When we first meet Sy he has been hauled into police custody for an apparently terrible crime.

As he is questioned, he tells his story in flashback. Having worked in the photo development lab of a huge anonymous store for years, he has gradually become obsessed with the snapshots of one family, the Yorkins.

To wife Nina (Connie Nielsen) and son Jakob (Dylan Smith) he seems like a harmless, loveable eccentric, but the desperately lonely Sy wants nothing more than to join their seemingly perfect Yorkin family.

It is a fixation he takes to extremes by putting endless photos of them on the walls of his flat and fantasising about being a bigger part of their lives than simply "Sy the photo guy".

Creepy

But when his job comes under threat, and he discovers at the same time that husband Will Yorkin (Michael Vartan) is indulging in some extra-marital fun with a colleague, Sy's obsession spins dangerously out of control.

Director Mark Romanek, a veteran of music videos, has not made a film since his 1985 debut Static, but on the evidence of One Hour Photo his second film has been worth the wait.

While the film could have resorted to standard psycho-thriller histrionics, it is a far more subtle piece of work, with Romanek delivering some of the most startling visuals and imagery to grace a movie screen all year.

Williams' soulless workplace is filmed in bleached-out colours, while by contrast the Yorkins' home is decked out in warm, sunny tones, perfectly illustrating the life our protagonist is stuck with versus the life he wants to have.

Williams, meanwhile, acts better than he has for ages, delivering a surprisingly restrained performance which only serves to make Sy, who is clearly several frames short of a full roll, even creepier than he actually is.

The climax delivers thrills and tension without going over the top, but it is the very last shot of the film, unnerving and chilling as it is, that will linger in your memory long after the credits have rolled.

One Hour Photo goes on general release in the UK on Friday.