Seemingly inhabiting a space between himself and Tufnel, Guest cut straight to the quick when confronted with the question fans had been asking for years: no, Spinal Tap were not based on any one rock band from the '70s or early '80s.

Of course, he always says this, and no one ever believes him.

Everyone who has ever been in a band can recognise the bizarre, sometimes surreal moments that occur on tour, even at the lower echelons of stardom. In fact, the humour is so close to the truth that when the film came out, many rock stars thought they knew who it was based on: themselves. "At the time, every band we met would say, 'It's about us, you're doing us.' Literally dozens of bands came up to me and said that," Guest says.

"People said, for instance, I was doing [British guitarist] Jeff Beck, and then Jeff Beck himself said I was doing him. I said 'no, we're not doing you.' We didn't do anyone specific, as it was more interesting to invent these people from the ground up. But there obviously has been some overlap with famous people," Guest admits.

is set in 1982, as the band's popularity starts to dive. Formed in the '60s, they had initial success with the folky psychedelic , before changing their musical style for the hard rock boom of the early '70s. But by the '80s they are going out of fashion. Ignored by their record label, they go on a tour which sees them drop from playing decent-sized venues to a disinterested audience at an army base.

Tension rises between the two founder members, Tufnel and David St Hubbins (McKean), a situation that's compounded by the latter's meddling girlfriend.

The hilarity is in the situations. The band get lost in the labyrinthine passages behind a stage, and can't find the stage door. Bassist Derek Smalls (Shearer) is caught inside a womb-like stage prop and can't get out. And in a legendary scene, Tufnel claims that his amplifier is louder than any other because the dial has an "11" on it, whereas most amps only go up to "10".

The film wasn't based on specific people but it was modelled on a specific type of band, Guest says, leaving the audience to fill in the blanks: Uriah Heep? Yes? Genesis?

"There were some pretentious bands, and a lot of them favoured themselves as great artists. We thought that was amusing," is all he'll say.

Spinal Tap began as a one-off joke for the 1979 pilot for a comedy called , starring Reiner and featuring a mock promotional video for the song . Guest and McKean used to write songs together - McKean apparently wrote love songs, while Guest, a bluegrass fan, wrote folk-y numbers, and that formed the basis for the TV skit.

"A few years after the TV appearance, we remembered how much fun it was, and decided to do it again. We were given money to write a script, but we quickly realised we couldn't really write down what we wanted to do, and it would have to be done in a more spontaneous way. So we took the money that they gave us, and started making the actual movie. The money dried up, and we shopped the 20 minutes we had made around Los Angeles. Everyone we showed it to said, 'What's this?' We said, 'It's a comedy.' They all replied, 'No it isn't'," Guest recalls.

Reiner, who plays documentary maker Marty Di Bergi in the film (probably a reference to Martin Scorsese, who directed The Band's ) finally got the money to make the movie. After shooting, he spent two years editing down 50 hours of footage.

A lot of good gags had to be excised, says Guest, but "Rob did a great job. It became evident that even if something was funny, it had to serve the story to be included. It's quite a serious story about the break-up of an old friendship. So things came and went in the script." (A four-hour bootleg version, which contains many of these excised scenes, is popular with hardcore fans.)

The film is often described as a faux documentary. But as far as the filmmakers were concerned, was a straight comedy film. "Some journalist called it a mockumentary," Guest says. "But we never liked that term. Still, it was the first movie done in that style."

A lot of the scenes did end up being improvised, as originally intended, Guest says. "It's not something that happens very often in the world of film. We wrote an outline that delineated all the scenes, and the complexities of the movie, and we shot from that. There was no rehearsal. There were specific jokes that we had to get in to make it work. But apart from that, it was all on-camera improvisation."

The film works because the band can actually play their instruments, and are able to put on a terrific live show. Songs such as , , and lampoon heavy rock tracks from the '70s and early '80s.

"We wanted to write the songs as we were musicians," Guest says. "It was a challenge, as they had to be funny, but plausible. We realised that if the songs didn't work, the film wouldn't work, so we had to find a balance between stupid concepts and workable songs."

Did he think they succeeded? "Well, we've been playing these songs live for over 25 years, so I think we did," he says.

In New York, Guest took in the festival screening of . It was the first time he had viewed the film in 12 years, and he was pleasantly surprised. "I've only seen it a handful of times, and I liked watching it just now, actually. I was surprised, as I thought it would freak me out. But I enjoyed it, and I remembered what it was like to make, and the fun we had doing it."