In a drab building in central Scotland, one afternoon in the armpit of winter, an actor who looks a lot like nice-guy James McAvoy is persuading a room full of blokes to - I'm paraphrasing here - photocopy their privates.
"Come on!" he roars, all pouchy of eyes, gingery of beard and 80s of suit.
A Christmas hat is jammed on his head, a bottle in his hand, as he leers malevolently in their faces. His beery challenge: where's their party spirit? Are they men or are they mice? They are, in fact, neither. They're coppers in possibly the worst police station in the world and, true to herd-like form, they duly follow his boorishly charismatic lead.
Now the party can really start. Within minutes our "hero" and a secretary are using the photocopier room for altogether different purposes.
"I hated that scene!" McAvoy is telling me, with feeling. "Is that the day you came? Aw shit ..."
It is a sunny day in London, spring 2013. As we talk on the balcony of a photo studio, 16 months have passed since I watched the Glaswegian go through his paces on takes four, five, six and seven of scene 111 of his new film, Filth.
"I was so gutted about that day," McAvoy continues, grumbling. "That scene always felt a bit 'what the f*** is going on?"' Adapted from Trainspotting author Irvine Welsh's 1998 novel, one of the darker books in his always-forbidding oeuvre, Filth is the story of a copper so bent even Uri Geller would be impressed.
Directed and adapted by newcomer Jon S. Baird, it is a gruelling, hilarious, scabrous, Edinburgh-set picaresque about corrupt police, friend-on-friend depravity, mental illness and cross-dressing.
As un-PC Bruce Robertson - a drug-taking, colleague-shafting, auto-asphyxiation-loving swine - McAvoy is a sensation, though he's never looked rougher. He's even required to sexually threaten his actress sister Joy, four years his junior, here cast as a gang member.
It's the first time the McAvoy siblings - raised on a social housing estate by their grandparents after their parents split when James was 7 - have acted together. "Apart," he chuckles, "from pretending I hadn't beaten her up when we were kids."
We've not seen McAvoy in such a pungent role before. "I don't think it pulls any punches," he muses. The film was so challenging that Baird had to scrape together finance, including from Sweden and Belgium. Twenty-six producers are listed in Filth's credits, and the profusion of creative opinions led McAvoy to take a pay cut in return for his own producing role to give Baird extra leverage and stay true to the film he wanted to make.
McAvoy isn't overstating the X-rated credentials of the film - which boasts an excellent supporting cast including Eddie Marsan, Jim Broadbent and Imogen Poots - nor the procedural challenges involved in shepherding Filth to the screen.
McAvoy agrees, with evident satisfaction, that Filth is "bonkers ... It asks a lot of the audience, and there will be people who walk out of the cinema, I'm sure of it. And there will be people who don't understand it and get totally lost. But for those who get it, I think they'll really love it".
All told, it's just the kind of role relished by a 34-year-old actor forcefully pushing his career in new directions.
As we talk, McAvoy is still processing another tough performance: his Olivier Award-nominated, 88-show portrayal of Macbeth at London's Trafalgar Studios, which closed three days previously.
At 1.7m tall he had attacked the role with gusto, amplifying Shakespeare's warrior king's battle-hardened vigour and raging nightly across the dystopian set.
Macbeth's lengthy run has taken its toll. "I've broken my thumb," McAvoy says, waggling a horribly swollen appendage. "I got hit by somebody's axe. I got stitches in my eye 'cause somebody hit me with their machete," he adds with something like pride. "So I was ready to finish physically but I am kinda bereft, really. I'm just missing being Macbeth and making people shit themselves nightly, and making people in the audience faint - which was getting really good fun. We were getting quite good at making people faint."
McAvoy has only a few more days of freedom before he flies to Montreal for the epic X-Men: Days of Future Past shoot. The superhero franchise is a rare excursion into blockbuster territory for him. Is this the film in the original series where his character goes bald?
"I think I do, yeah, I'm waiting to see," he replies, hedging his bets - the star of a superhero film reveals plot spoilers at their peril.
Has he been fitted with a skullcap? "Oh f***, mate, don't!" he winces. "I think I'd rather just go proper baldy - shave it and wear a wig for when I don't need to be bald." It worked for Jessie J, I say.
"Yeah, but she's fit," he grins. "I'm getting on a bit. I'm not gonna look good bald ... But yeah, it's gonna be good. We've got an amazing bunch of people - we've got [Michael] Fassbender back, Nick Hoult back, Jennifer Lawrence back - we all had a cracking time last time. And the old guys - I don't mean old people," he adds hastily, laughing, "I mean the people from the old movies."
Again he's "sworn to secrecy" as to how the younger incarnations of the original X-Men cast interact with Patrick Stewart, Ian McKellen et al. "Ah cannae talk about it," he grins, going full Glaswegian for a minute, "but we will get to hang out. And there's going to be four Macbeths! Me, McKellen, Stewart - and Fassbender's gonna do it on film, apparently, some time at the end of this year. So I'm sure that's gonna be a right old giggle."
It's a long shoot, but presumably a lighter role for McAvoy? A lot of touching his temples and pretending to read minds?
"Yeah," he nods. "It'll be a doddle, man, compared to what I've just been doing.
"There is some fairly emotional and mental draining stuff in this new one - 'cause you've kinda got to f*** Charles up a little bit. But yeah," he says with a thin smile, "you're not gonna go to the places you go to in Filth."
Who: James McAvoy
What: Filth, the outrageous film of a rogue Edinburgh policeman from the Irvine Welsh novel.
When: At cinemas from today