Darren Boyd and Rebekah Staton: Spy's talented comedy couple

Classically trained Boyd began his career in the West End in French revolution sing-along Les Miserables, but aside from the odd ditty on Smack The Pony back in the day, his vocal talent has remained dormant over the past few years. When it came to it, he discovered he was a tad rusty. "It’s like thinking it would be a good idea to bench-press an extraordinarily heavy weight having not been to the gym for ten years, because you once did that weight," he tells RadioTimes.com. "And you sort of lay back and think: ‘Well I might get it off the stand.’"

But the gamble has paid off. "He’s going to blow your mind, because he’s amazing," gushes Staton, who plays Tim’s fellow MI5 agent and love interest Caitlin - their will-they-won't-they romance being the other highlight of the Christmas episode. Boyd says returning to singing showed him what he had been missing. He muses: "It was a really nice place to revisit. It took me back to a feeling which is indescribable and unrivalled, and though I love what I do there’s something about that that just feels good. When you start tuning back into it you realise that it’s actually maybe a really large part of who you are.”

But for now, Boyd is limiting himself to private concerts for his baby daughter Eliza, who he’s been singing to “ever since she was in the tummy”: “We make up songs as well, I mean, I make up songs. There’s a song for when we change her nappies that involves the wet wipe and there’s one about friends.”

Boyd is keen to play down his singing on Spy, pointing out that it's only a few lines and isn't central to the episode – which is true, but his voice is genuinely impressive. Of course, one person who isn't impressed at all is Tim's precocious and rather annoying ten-year-old son Marcus, played by Jude Wright. Boyd confirms that, off-screen, Wright is nothing like the icy smart-alec he so brilliantly plays in the show – Boyd is subjected to young Wright’s much more normal, boyish antics, which have taken a somewhat morbid turn: "In between takes he’s much more interested in reaching in and taking out your innards or holding a gun to your nostril. He’s just finding different ways of miming disemboweling you, with sound effects, with slow motion…" Boyd sighs, with the mock-resignation of a real weary father.

When Spy series two wrapped, Boyd reprised the paternal role for Sky1’s Little Crackers. The silent-movie style semi-autobiographical tale that he also wrote and directed aired on Friday. Talking to us a few weeks ahead of the shoot, Boyd seemed satisfied that its production was under control: “We’ve don’t have time, but we have time, if you see what I mean. These things often thrive on that collected slightly panicked energy.”

Boyd is “incredibly excited" about the short film, which sees him join a star-studded list of British comedy actors, including Joanna Lumley, Katy Brand and Dylan Moran, invited to recreate a moment of their youth. In Boyd’s case it’s not strictly autobiographical, but the theme is based on real life: “It’s about the moment of really being noticed, that validating moment when the kid wants the parents’ approval and gets it, that moment of really connecting,” he says.

Vicar’s daughter Staton knows what it’s like to be doted on by her father. The 31-year-old recounts how, in his sermons, her dad would proudly announce from the pulpit any shows she had on. Fine for the literary classic Tess of the d’Urbervilles, say, but what about legendarily filthy comedy Pulling? “That slightly backfired,” she laughs, “I think the congregation was a bit shocked about Pulling!”

In the BBC3 hit, Staton was overweight and awkward Louise, a far cry from her Spy character Caitlin, the focal point of a love triangle and an undeniably sexy and sophisticated role. Staton explains how she “transformed” herself for Spy and admits that a slimmer waistline has made adapting herself easier: “You get down to the gym, change your physicality a bit, and I think that helps people to see you differently and therefore cast you differently.”

Staton says she has never let herself get comfortable in the acting industry, despite her rapidly rising profile, and instead has tested herself with eclectic and challenging roles. She puts this down to having received a scholarship to attend prestigious acting school Rada. The grant amounted to about £40,000. “There comes a point when you think: ‘I’ve got to show them I was worth that investment. I still feel like that now, 10, 11 years on.”

Pushing boundaries while having a laugh – and revealing a few hidden talents along the way - could be the secret of Spy's success.

The Spy Christmas special airs tonight on Sky1 at 8pm