“I remember sitting on my bed one night and reading it through,” he recalls. What is more, it was straightforward fascination rather than professional curiosity that kept him reading. “I don’t want to know the character they want me to look at when I read a script,” he says. “I just like to read it as a story. I thought it was a great script and that Prot was a wonderful character. It took time for other parts of the project to fall in place, but it was worth the wait.”

With a film generating as much enthusiasm as K-PAX - released in the US on October 26, it has been selected for the prestigious closing-night gala of the London Film Festival, whose director, Adrian Wootton, describes it as “truly astonishing” - it is easy to forget the struggle Gordon and fellow producers Lloyd Levin and Robert F Colesberry had to bring it to the screen.

In a climate where short attention spans and instantly predictable endings seem to rule, a story as unconventional as K-PAX was never going to be an easy ride. “We ended up with an excellent screenplay, and it still took six years to get the film made,” says Gordon. “But once Kevin saw the script and became interested in playing Prot, the project moved forward much more quickly.”

Powell with his wife, Rachel, played by Mary McCormack.

K-PAX finally reached the screen thanks to Intermedia, the most active and successful of the several companies which have recently drawn on the influx of German capital into English-speaking film production. Based in Los Angeles, London and Munich and headed by international film-scene veterans Guy East, Nigel Sinclair and Moritz Borman, Intermedia’s current slate includes submarine drama K-19; The Widowmaker, starring Harrison Ford; British wartime thriller Enigma, starring Kate Winslet; and the upcoming The Quiet American, starring Michael Caine and Brendan Fraser. For K-PAX - which is released in the US by Universal - filming began on location in and around New York almost exactly a year ago (on November 13, 2000), with a cast that also includes Alfre Woodard and Mary McCormack. Behind-the-camera talent comprises John Mathieson (who shot Gladiator), and production designer John Beard, who worked on Dove with Softley as well as on films by Terry Gilliam and Nic Roeg.

Prot is treated by Dr Mark Powell (Jeff Bridges), who begins to wonder just how delusional his delusions are.

The story is this: Spacey’s character is arrested one day in New York’s Grand Central Station in the aftermath of a mugging. He politely explains to the arresting officers that his name is Prot, and that he comes from a planet called K-PAX which is a thousand light years from Earth. He is, he says, on a fact-finding mission and, all things considered, earth is quite pleasant - with the exception of the light, which he finds rather bright. Either way, he will be returning home at the end of the summer to report his findings.

New York’s finest have heard many variations on this story before and have little hesitation in committing Prot to the local psychiatric hospital, where he is placed under the care of Dr Mark Powell (Bridges). Powell, too, has heard this kind of thing before, and patiently begins treatment.