'It seemed silly not to take a chance'

"Every time I got a bruise or a broken tooth, rather than get the nurse in, it was, 'Get the cameras in, so we can show you're going through it!'" recalls the 54-year-old.

That's because in his latest incarnation, the man known to millions as Mr Darcy, from both the Pride And Prejudice TV series and Bridget Jones movies, has turned action spy and martial arts expert, in the comic book adaptation Kingsman: The Secret Service.

"When we were shooting it, the stunt guy would say, 'Let's get the B-roll in to film some of this, because nobody's going to believe it'."

Firth, looking handsome in a dark suit and black-rimmed glasses, couldn't believe it either, and is more than aware that his many fans will see it as an unexpected role.

"It was unexpected for me too, but it sparked my interest immediately because of that," admits the Hampshire-born actor, who was already intrigued by the director Matthew Vaughn, who also helmed Kick-Ass and X-Men: First Class.

"I think he's a one-off. There was no script yet, it was a work in progress and he wasn't ready to show it to me, so I just went to hear what he had to say and I found him very compelling," reveals Firth, who was Oscar nominated for Tom Ford's A Single Man in 2010, and walked away with the gold statuette for The King's Speech the following year.

"Something in me just went, 'Why not?' It's so unusual for me, that it seemed silly not to take a chance."

Early on, Vaughn was "full of dire warning" of the extent of training he'd be required to do. "And how much it would hurt and how much I was going to hate him by the end of it, and was I up for that," Firth adds.

"It was three hours a day for about six months, and started off in my back garden with a squat, lunge and somersaults, and then the choreography kicked in. I was learning to use parts of my body that I'd never used - I didn't even know they existed. It was painful, but I warmed to it and found it exhilarating," he continues.

"This wasn't the stuff I was doing when I was young. I wasn't one of the more athletic drama students. My recreation was a lot more sedate in my youth, so it was a chance in my 50s to do something completely different, and go into a bit of a physical world for a while."

Bar one stunt, involving a rig, it's all Firth on screen. "I said that to somebody, and they looked at me with horrified disappointment that I would tell people such an egregious lie," he notes, smiling.

"One of the reasons I feel so driven to tell people, is that I won't have gone through all that pain for nothing."

The effort definitely paid off. Not only did Livia, his Italian wife of 17 years, appreciate her newly honed husband ("It got noticed!" he confesses, laughing), but he looks a natural in the role of Harry, an impeccably suave spy and member of the secret organisation, known as Kingsman.

Harry recruits street kid Eggsy, played by newcomer Taron Egerton, and trains him up just as a global threat emerges from a twisted tech genius, brought to life by Samuel L. Jackson. A pastiche of the spy movies we all know and love, Firth acknowledges there's a lot of violence - but "it's wildly implausible".

"It's like a provocative pantomime, and in some ways I think it's sending up the extremity. I can't imagine how anyone would emulate it, because you'd need to be trained to do that stuff. It's closer to what you get in Monty Python, than it is to the realistic films about the Middle East."

He hasn't decided whether he'll allow his two sons - Luca, 13 and Matteo, 11 - to watch it (he also has a 24-year-old son from a previous relationship with actress Meg Tilly).

"I'm not going to publicly recommend this to anyone under 15. But it's different, because my children saw me train, and went to set and saw me backstage and saw me in the workplace, so they have a very different perspective."

And whether or not people believe it to be gratuitously violent is "a valid conversation" to be had, Firth adds.

"There are fights in Punch And Judy and Shakespeare, and there's violence in war films and zombie films. There's violence of all types, and unless you believe they should be extracted from all storytelling, then it has to become a conversation about where your own threshold is," he remarks.

"Whatever that threshold is, Matthew likes to go towards the edge of it, and sit there and be provocative and outrageous but without, I think, any intention to horrify. I think the violence in it is comic in tone, satirical and completely silly."

There'll be action of a different sort, when Firth starts filming Deep Water with The Theory Of Everything's director James Marsh this spring. "It's about Donald Crowhurst, who was a participant in the round the world yacht race in 1968," he explains. "But nothing's ever set in stone until it's ready to go."

Genius, on the other hand, a chronicle in which he plays Max Perkins, the book editor at renowned publisher Scribner, who oversaw works by F. Scott Fitzgerald and Ernest Hemingway, is in the can.

The movie marks his third collaboration with Nicole Kidman, following The Railway Man and Before I Go Sleep. He grins when asked if they'll work together again. "Inevitably," Firth replies. "I think we'll go for the multiple box-sets."

EXTRA TIME - BEHIND THE SCENES

:: Kingsman: The Secret Service was written by Matthew Vaughn and Jonathan Ross' wife Jane Goldman, and is based on the comic book by Mark Millar, after he and Vaughn conceived of the idea on the set of Kick-Ass.

:: Vaughn was inspired after reading a newspaper article about the fact the Dr. No director Terence Young cast Sean Connery against the wishes of 007 author Ian Fleming, and realised he had to turn him into a gentleman spy.

:: More than 60 young men were screen-tested before Taron Egerton, from Aberystwyth, Wales, and fresh out of drama school, was cast in the role of street kid Eggsy.

:: Sofia Boutella, who plays bad girl Gazelle, established her career as a dancer, leading in three of Madonna's tours, and as a brand ambassador for Nike.

:: Martial artist and action choreographer Brad Allan, who worked with Vaughn on Kick-Ass, coordinated the fight sequences with a team that included a parkour champion and a break dancer.

:: Kingsman: The Secret Service is released on Thursday, January 29