"Everything about it was technically impossible or really, really impractical," says Linklater, a boyish 54-year-old. To begin with, the funding required an annual cash injection from a financier willing to wait 12 years before seeing a return on the investment. When he first met with producers, they simply couldn't "wrap their head around it", he says. "They said, 'We're not a bank. We can't just give money out'."

Eventually, IFC Films committed, offering US$200,000 a year - or US$2.4 million in total. Those that backed away must be cursing their decision. Its US$43 million worldwide box office takings thus far may be nowhere near the US$131 million gross of Linklater's 2003 comedy , but given its low key nature, it's a remarkable haul.

On many critics' year-end top 10 lists, has already scooped major prizes at the Los Angeles and New York Film Critics' awards, and been nominated for five Golden Globes, including best director. It will certainly be in the running when the Oscar nominations are announced later this month.

Hawke, who began his collaboration with Linklater in 1995 with , admits he's baffled by the film's success. "You really hope people understand where you're coming from," the 44-year-old actor says. "I thought [Linklater's animated film] was brilliant, and that didn't really find an audience. I expected the same thing to happen to , so I'm in shock about it. But it's a movie about growing up and that's something we can all relate to. I guess it's more accessible than some of Richard's other work."

It was a film that required remarkable commitment from the actors. Not that this proved too difficult, says Arquette. "I always looked forward to going back. The crew and the actors always had this excited attitude on set every year. It was, 'Can you believe we're making this? It's so cool'." Even so, the actress, who spent seven years on the TV show during this time, had to negotiate time off to fit into her schedule. "I would beg them for this time," she says.

As logistically complex as it was working around the busy lives of actors like Arquette and Hawke, that was nothing compared to selecting the right youngster to play Mason. "I met a lot of kids back then, and he seemed like the one to bet on," says Linklater. "He seemed the most interesting. I liked the way he thought, and just expressed himself about what was going on with him, at age six. But he also had very artistic, cool parents. And I thought 'OK, he'll have the family support.' Because this is a huge ask. You can't even legally ask anyone to do anything for 12 years."

Thankfully, Coltrane diligently stuck to his task - remarkable when you consider that the film took him through his teenage years, a time when he could easily have rebelled against it. "The time when I could've turned that corner and become jaded by it, or fed up with the experience, is the time I fell in love with it," says Coltrane, who's now 20. "That was right when I was becoming more a part of the process of creating the film and crafting it."

Even with the total devotion of Coltrane and his co-stars, Linklater had no contingency should one of the actors get sick, or worse. "There was no escape plan. Those thoughts vaguely cross your mind, but you don't really go there," he says. "We had great faith in the process. I think everyone believed in the core mission of the film. I was just trying to make it work - that was tough enough, so I didn't imagine dark scenarios about what could happen. We all go through life with that. The phone can ring any minute, and we can get the worst news of our lives."

While the risk-intensive nature of gives it energy, the magic comes from the breathtaking patience it took to make the film. Like a living photo album, subtly cut together by editor Sandra Adair, the transitions as the cast grow up and grow older are astonishing to see - although maybe not if you're Arquette, who was 34 when she started the project. "Visually, it's pretty brutalising, I'll tell you that right now," she groans. "But emotionally it's exactly what we all wanted."

Raising questions about evolution and ageing, it could keep the nature versus nurture debate burning for years. How much of Coltrane's own personality growing up was shaped by his experiences as Mason, and vice versa? Linklater, for example, encouraged his interest in photography; when he took to it, they wove it into Mason's character. "It's so eerie to watch," says Coltrane. "It's this different character, and a lot of his experiences were pretty different than mine were growing up, but his personality is also very much mine."

Much in the way Linklater and Hawke drew on their own lives when creating the series, this is a similar collage, blending various experiences into one living, breathing whole. "I guess my personality is in there," shrugs Linklater. "I always said this was a combination of my childhood memories, Ellar's childhood, Ethan's childhood, and Patricia's."

Whatever the influences, it's a film that simply cannot be missed.