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 — Xerox their cocks.

"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 — dully — 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!" James McAvoy is telling me, with feeling. "Is that the day you came? Aw shit ..."