Lenny Abrahamson, the director of ‘Adam Paul’, has cast the Irish star to play the much-mourned Frank Sidebottom, alongside Domhnall Gleeson and, as a theremin virtuoso, Maggie Gyllenhaal, and they’ve been filming secretly in Co Wicklow

Something unexpected is happening at a secret location in Wicklow. Negotiate your way past a vertiginous mountain pass, stumble across an impossibly idyllic babbling brook and, if you’ve got the right brook and the correct pass, you will encounter an incongruous crop of sub-Bavarian chalets. A bizarre party is lined up waving goodbye to unseen guests. One of their number, tall beneath a woolly Russian hat, looks a great deal like Maggie Gyllenhaal. Another is surely Domhnall Gleeson. A third wears a giant, bladder-shaped head with cold comical eyes. The head comes off. Good grief! It’s Michael Fassbender.

We are on the set of the latest film from Lenny Abrahamson. It took five years for the Irish director, lionised for his debut, Adam Paul, to follow up the slippery, moving Garage. After performing strongly at the Irish box office, What Richard Did, his tale of tragic goings on among the Irish middle classes, went on to receive staggeringly positive reviews from British critics. Happily, it seems we will not have to wait quite so long for his fourth piece.

Based on a script by the journalist Jon Ronson and the busy screenwriter Peter Straughan, the baldly titled Frank traces its origins – distant origins, as we shall hear – to Ronson’s relationship with the much-mourned postpunk entertainer Frank Sidebottom.

A creation of the late Chris Sievey, Frank, decked out in that scary papier-mache head, gained minor celebrity by singing such odd, unsettlingly naive tunes as Anarchy in Timperley and Xmas is Really Fantastic.

I have, however, barely set foot on the muddy track before it is explained that the film is no sort of Sidebottom biopic (if such a thing were even possible).

“The vague story is about a guy called John who comes across this eccentric American band,” Ed Guiney, Abrahamson’s inseparable producer, explains. “He then goes back to his normal life and is called to meet them at a gas station. He joins them, and they come here to record a weird album and later make their way to Austin.”

Whereas Sievey made much of his northern English roots, this Frank is an American of the current avant-garde school. Sidebottom was a stage act. The disturbed hero of Abrahamson’s film seems reluctant to ever remove his false head.

Hang on a moment. They’ve hired the world’s hottest actor for a role that requires his face to be concealed throughout. This sounds properly weird.

During a break in filming, Fassbender, slim and graceful in a plain autumnal cardigan, makes his way towards an outlying chalet to explain himself.

“Lenny didn’t have to talk me into it,” he says. “I read the script and thought it was hilarious. It’s so very different to everything else you read. Half of the time you can guess what will happen from page 10. That was definitely not the case with this.”

So let’s put this to bed. This is not the Frank Sidebottom story. “I have no idea,” he says. “I knew nothing about him until I read the script. Then I began Googling stuff about him. I had no idea what he was about. Everything is in the script.”

Distant background

Ronson was closely involved with Sidebottom. He briefly played with his band and, when Sievey died in poverty, helped to raise money for his funeral. Did Fassbender not ask Ronson about the film’s distant background?

“No, I don’t like to talk to writers about their writing,” he says. “I really don’t. I think that’s a cheat. It’s taking the easy route – getting the answers directly. They write what they write, and then it’s handed over to me and the director and everybody else. The work then doesn’t get set within one person’s firm idea.”

You could argue that Fassbender, raised in Kerry to a German dad, is one of those actors who really does justify the overused metaphor that links certain actors with chameleons. The intense, wasting protagonist of Hunger, Steve McQueen’s study of Bobby Sands, came across as a very different figure from the withdrawn Rochester in Jane Eyre. The cold cyborg in Prometheus was something else again. Still, his face remains a valuable piece of cinematic real estate. He must have had reservations about covering it up.

“No, I am going to do this all the time from now on,” he says with a laugh. “I am going to wear the head for everything. I love that idea. Ha ha! But, no, it really is part of the story.”

We trump through a temporary hurricane to watch the completion of the scene. Somebody has mentioned that Gyllenhaal’s family is visiting. Sure enough, her young daughter, a minute, near-perfect facsimile of mum, joins us to huddle around a monitor at the rear of the shack. Then Dad appears. It’s the charismatic, currently heavily bearded actor Peter Sarsgaard.

The head is replaced on the willing star. Some more waving takes place.

Later, ploughing through a very decent lunch in the inevitable double-decker catering bus, Abrahamson explains how they tailored the scene to Fassbender’s special abilities. The band (whose name is comically unpronounceable), having dissuaded a group of German tourists from evicting them from their recording space, are sending the travellers on their way. The opportunity to have Fassbender deliver a few lines of his second language was not to be overlooked.

Head concealed

I don’t want to go on about this. But what is Abrahamson thinking by keeping the Fassbender head concealed?

“I know. Everybody is trying to get Michael in front of a camera,” he says. “We have got Michael in front of a camera, and we cover him up. Actually, I love gambits like that. Casting Pat Shortt in Garage, a serious film, was a bit like that. What are you doing? But it’s appealing to people because everybody wants to know how we’re playing it. It becomes part of the story of the film.”

He goes on to explain how Fassbender’s physicality overpowers the disguise. By lighting the head in different ways, Abrahamson allows that apparently impassive globe to convey very different emotions. “It shows the power of the medium,” he says.

It seems as if the writers of Frank liked Adam Paul, Abrahamson’s bleak comedy about Dublin junkies, and made sure that their script got to the director. This makes sense. From what I’ve heard so far, Frank seems, like Abrahamson’s debut, to straddle the frontier between the comic and the sinister.

John, played by Gleeson, finds himself part of an uneasy collective that makes a sound every bit as unusual as – many other comparisons are bandied around – those produced by the blues anarchist Captain Beefheart and the outsider troubadour Daniel Johnston. (Stephen Rennicks, another long-term collaborator, has been working hard on the music with the director.)

“Yeah, this is much more like Adam Paul than anything I have done since,” Abrahamson says. “It doesn’t quite have the bleakness of Adam Paul. It’s a warmer film. One thing that I liked about Adam Paul, though, was that it moved from quite broad farce to something real and still felt like the same film. I think if people are moved by the film it will creep up on them. There is a continuity between the first two films and this.”

It sounds as if Gleeson’s character may be looking through the audience’s eyes. He plays the relatively normal outsider who tries desperately to process the rampaging weirdness. As events progress, he develops an increasingly close relationship with the disturbed Frank and experiences ever greater hostility from Gyllenhaal’s theremin virtuoso.

Sheltering in a Portakabin at the outer edges of the encampment, Gleeson muses on his apparent role as a stand-in for the paying punter. “I think so,” he says. “He’s a bit lost with the band for a while – which we will be also. There’s a bit of crossover there. But the difficulty with being the eyes of the audience is that the character can disappear. But Lenny has been very careful about making sure that doesn’t happen.”

The conversation with Gleeson is perhaps the oddest part of an odd day. Recently seen in Joe Wright’s version of Anna Karenina and James Marsh’s fine Northern Irish thriller Shadow Dancer, Domhnall – son of Brendan, of course – is almost as head-spinningly busy as Fassbender.

He remains, however, unaffected and amiable. It’s the voice that’s disconcerting. Who’s this English bloke? Gleeson has decided to stick with the accent between takes. The fact that he’s nailed it so effectively makes the conversation even more peculiar.

“It’s definitely not a character thing,” he says, sweeping aside accusations of preciousness. “I have done it for the last few films, though. I did it for Shadow Dancer because I had heard so many bad Northern Irish accents. If you don’t the vowels do start to come out.” He laughs and swills some coffee. “It is a bit arsey, I know.”

Culture shock

As Gleeson happily admits, the cast have undergone a fairly serious case of culture shock over the past few weeks. Shooting on Frank began in New Mexico and then speedily moved to this overpoweringly gorgeous, bracingly damp corner of Ireland. The home country plays itself and England. Albuquerque stands in for Austin. Abrahamson could hardly have induced a greater jolt if he’d dragged the poor souls from Kinshasa to Vladivostok.

Still, Gyllenhaal appears to be bearing up quite nicely. She refuses to complain. She misses the chance to make any diva noises. How disappointing.

“It’s been good but wild,” she agrees. “All of a sudden we’re here and there are different characters, weird accents, Guinness. Ha ha! You could not experience greater differences in terrain. From dry, dry sun to wet, wet cloudiness, and those things really affect the experience. And, of course, it’s such a very weird movie. It’s so unusual.”

Which brings us back where we started. Whatever else happens with Frank, even the most unforgiving observer would have to admit that Abrahamson and his team are not afraid of edging into fresh, unexpected, weird territory. We will, most likely, have to wait some time before the picture emerges. Guiney is cautious about discussing release dates, but no official sighting of the head is likely before the end of the year. Wave goodbye until the seasons close in once more.

Darkness looms and we trundle our way back up the precarious path towards the relative normality of rural Wicklow. Leaving Shangri-La must have been a little like this. If Shangri-La had proper catering.