There is also something heartening about Frank's spectacularly un-commercial decision to cast a face as recognisable as Fassbender's, then conceal it beneath a fake head. In the movie, Frank even wears the head in the shower.

"It's pretty comfortable now. I'm used to it," explains Fassbender during a break. He has taken the head off and it really is him beneath it. "It was kind of weird in the beginning, the way it sits on your head. You feel a little bit vulnerable because you've not much peripheral vision. You can only see straight in front of you. You can't hear that well either. But it's quite liberating as well, in the way that masks are. It's amazing how many emotions that face manages to convey. It can look vulnerable, it can look sinister, at times it looks confused."

Before this, Fassbender was shooting in Texas with Terrence Malick. A few months hence, he will receive an Oscar nomination for 12 Years A Slave. Why is he doing Frank? "The script just made me laugh out loud," he replies, admitting he had never heard of Frank Sidebottom. "I thought there were also poignant moments there. It's a very unusual story, a real standalone. If it's refreshing to read, it's hopefully refreshing to watch."

There's another way in which Frank boosts its alternative credentials, which really is refreshing: the band is a real band. The actors actually play the music you hear in the film. That explains why the Soronprfbs' drummer is Carla Azar, a sometime member of Jack White's band (you can tell just by that half a fill that she's a professional). The bass player is Francois Civil, a French actor/musician. Completing the lineup is Maggie Gyllenhaal on vocals, Moog and theremin. The latter is not an easy instrument to learn, she says. "It's basically like playing electricity." Gyllenhaal plays Frank's protective girlfriend, Carla. Acting opposite an expressionless head is no big deal. "It's not entirely foreign to me. I've had experiences like that in my life."

Like many actors, Fassbender, who sings and plays guitar and keyboards in the film, admits he once harboured fantasies of rock stardom. "I wouldn't call it a band," he says, recalling his efforts as a 17-year-old. "It was just me and another guy on guitar. We could never find a drummer or a bass player. We never even got as far as having a name. We were both called Michael so we could have been Michael and Michael." No wonder he turned to acting.

Before coming here to pretend to rehearse, they spent three weeks actually rehearsing in Dublin, under the guidance of Abrahamson and his composer Stephen Rennicks. It wasn't quite a Beefheart-style drilling, but they threw themselves into it, says Gleeson. "Every time Stephen and Lenny stepped out of the room, Michael would pick a song that we all had to try and play while he sang over it." The cast are enjoying being a band so much, they're talking about going on tour when the film's finished.

So what do the Soronprfbs sound like? Abrahamson's vision was "pathologically eclectic". Suggestions from the cast include Captain Beefheart, Can, Sigur Ros and the Beta Band. "Somebody said one song sounded like Jim Morrison," says Fassbender, "but then the next one sounded like the Sex Pistols. We're never supposed to be good pop music, we're supposed to be odd music that strikes a beautiful chord in some kind of way, but remaining sort of … not shit music exactly, but let's say … hard to market."

Is there a difference between genuinely uncommercial and marketably uncommercial? Is it the difference between Frank Sidebottom and Frank the movie? Even a mid-budget movie costs a lot of money to make, and needs to make it back whereas financial success was practically an obstacle to artistic achievement for most of the outsider musicians Frank celebrates. Sievey died practically penniless. He only received a proper funeral thanks to a last-minute fundraising appeal by fans. Who knows what he would have made of Frank the movie. Doubtless he would be flattered, but he would be even happier to know that a life-size statue of Frank Sidebottom now stands in Timperley.