"Will you just get on with it?"
"It was a very slow burner, and it was never even scripted," laughs Firth, reflecting on the very British never-to-be-spoken-of love affair that captured the fans' imagination over ten series.
"It was just Nicola (Walker) and I kind of flirting and holding looks too long during takes, and they picked up on it, and then stretched it out as long as was humanly possible.
"Of course, the romantic in me wanted them to end up in the cottage in Suffolk and live happily ever after, but of course, looking at it realistically, that was never going to happen..." he drifts off with a sad smile, worthy of the enduring Harry Pearce.
Firth admits to mixed emotions on filming the final scenes of what had become a bedrock of British drama. Spooks may have become increasingly pyrotechnic and unbelievable during its ten-year run, but it nevertheless maintained a high production quality, for which Firth remains proud:
"I was glad that we were able to go out on a high and, despite the very operatic ending, managed to end on an upbeat note, back in the saddle..." Well, for Harry at least.
"But, I also felt nostalgia and sadness that ten happy years had come to a close. Filming the show was a world full of possibilities - difficulties on some days - but overall, a lot of fun and laughs. Particularly when you're dealing with that kind of subject matter, you've got to seek out some light relief in between, otherwise it could all get a bit dark."
After a long, mixed acting career, including runs on Broadway and success in LA, Firth still admits to being as surprised as anyone by Spooks' enduring claim on its fans' affections:
"It felt good, but not ten series' worth, necessarily. But if I knew what worked, I'd be running BBC Television. Every project starts with good intentions, but nobody really knows."
And after such a diverse acting history, does Firth mind the risk of typecasting that comes hand in hand with success on such a scale?
"Well, there are enormous benefits, particularly in middle age, of having such a steady gig," he reflects. "It's been very good to me in that respect, and professionally very fulfilling to be in something so popular and meaty. I've avoided episodic drama previously because I always thought it was a sell-out, but I'm very proud of this show - to be in it is fantastic."
The increasingly far-fetched dramatic reach of Spooks - with its core operators equally adept at code-breaking Iranian banking codes as they were at dressing up in their finery and speaking Russian at dinner parties, it seems - does beg the question of whether any real-life Security Servicers were involved, and what they thought of the show.
"One real-life spook came on board for our first series," remembers Firth. "He was meant to be our technical advisor, but every time I asked him a question, he’d tell me the answer was an official secret, so he soon proved dispensable.
"After that, we decided that we could make it all up anyway, so away we went."
What ensued was a radical approach to film-making, with scripts somehow staying abreast of such topical events as Osama Bin Laden's death. How did they manage?
"By not having any scripts," laughs Firth. "Stories were configured in a very random, haphazard fashion. Sometimes we'd start shooting episodes with no storyline, we didn't know where the story was going.
"I developed a catch-all performance with certain looks that would cater for any eventuality, and the brilliance of the editors and
scriptwriters came to the fore with them somehow culling together great TV out of all of this."
Firth, never a great fan of spy drama before his Spooks tenure (he was living in America when Tinker Tailor appeared on our small screens, and has yet to see it at the cinema), has nevertheless had his own eyes opened to the work of the security services:
"Even though we don’t really know what they get up to, it's certainly given me more pause for thought. Everything is certainly not what it seems. A road closed off for a gas leak is probably not a gas leak, and there are ways of controlling people... for the common good."
The grand finale of Spooks was left with the prospect of Harry Pearce's return - is this something that Firth would like?
"They've left it open, haven't they?" he muses, conspiratorially. "There's rumour of a film... it'd be great because the inbuilt audience means they wouldn't have to sell it. I suspect the BBC may realise that they had a pot of gold in Spooks and may want to reinvigorate it."
If they do, it will no doubt be Harry Pearce once again at the helm. Why does Firth think, his excellent acting aside, that his trench-coated very British warrior stole the show?
"He had perseverance, with qualities that are admirable, that you would like to see in your protector, a strong moral sense... most of the time...
"Living with him for that long, he must have changed me, although I'm not sure how. I think his better qualities I now aspire to, although they must be qualities within me, because that's what an actor has to reach into. But it has put them in bold print and made me more aware of them. I strive to be dignified in life now."
Spooks Series 10 DVD box set goes on sale today.
SLIDESHOW: The Spook's stars in action...