Rebel of an acting dynasty

Laurence Fox is sitting, like a giraffe in a box, in his tiny garret dressing room at London's Duchess Theatre. On one wall, brushing the top of his head, hang his costumes. His guitar is propped, ever so casually, against the wall opposite, within an inch of his outstretched feet. Long limbs draped like pipecleaners around his chair, he is glugging Tesco Exotic juice from the carton and wolfing Parma ham straight from the packet. "This is going to smell disgusting, by the way. Always does. Apologies."

It's not quite the glamorous, refined scene you might expect from this famous scion of an even more famous acting dynasty, one which has had patrician/ plummy-voiced/period roles sewn up for the past half century. Not that Fox, 34, son of James, nephew of Edward, cousin of Emilia and Freddie, subscribes to any of that. "I don't feel like we're part of an acting dynasty, whatever that means." But you definitely are. "But seeing as I've never not been, I don't know any better." He grins through a mouthful of ham.

His new role is also quite a departure from the Fox norm. Best known for playing cerebral sidekick Sgt Hathaway to salt-of-the-earth Inspector Lewis, period romantics (Gosford Park, Becoming Jane, A Room with a View) or posh officers (Colditz, Ultimate Force, The Last Drop), he is now playing a foul-mouthed, over-sexed squaddie. Joe is the central character in Our Boys, Jonathan Lewis's warts, wheelchairs-and-all play set in a military hospital in 1984; initially the ward joker, he gradually reveals the full, tragic extent of his PTSD.

In fact, Joe could be closer to the real "Lozza" than his typical strait-laced, stiff-upper-lip screen roles. He is, it turns out, a bit of a rebel. His method? "Have a pint, it'll be fine." Then there are the tattoos. Snaking wonkily down his left arm is "Mrs Fox 31.12.07", an indelible reminder of the day he married Billie Piper. There's a small W, for his son Winston, on the back of his neck. And, on his right shoulder, a giant broken cross and the number 139. "Which meant a great deal at the time," he says, looking at it, puzzled. "Psalm 139. 'Search me, God, and know my heart'. It's a good one."

He is a "vaguely lapsed" Christian, having been brought up in a god-fearing household. As members of the American evangelical church The Navigators, his parents would take Fox and his four siblings with them to Christian conferences. "I think the world is a better place for people that believe in God", he says. "I haven't quite squared that circle myself."

The opposite of impassive Hathaway, he is restless, silly, a bit of a motormouth. At the first opportunity, he ducks out of his dressing room to chainsmoke roll-ups on a tiny rusty balcony. "There's something quite nice about walking through a stage door and it all being a bit grotty and grimy", he says. "Seeing what happens. Live performances are always more fun."

The last time he did theatre was five years ago, when he starred in Treats at the Garrick. The on-stage drama was briefly overshadowed when he was arrested after a scuffle with a paparazzo at the stage door. He was released with a caution. More significantly, it was on the set of Treats that he met Piper. They married later that year and now have two children, Winston, three, and Eugene, five months. They live in a village in Sussex, 20 minutes down the road from Cheryl Cole and far away from the paparazzi. "If we lived in London they'd live outside and wait for you to drop your kid like poor old Peaches Geldof and then take their photo of it and vaguely insinuate that you're a terrible parent. They're scum, a lot of them."

Piper is also back on stage, currently in rehearsals at the National for The Effect, a new play from the writer of Enron, Lucy Prebble. For a few weeks, the couple will compete for audiences: do they compete over their careers? "It's not competitive but I feel like I've got to raise my game a bit. She's really good so it would be nice if I was really good too."

Surprisingly, given his genes, it took a while for Fox to find his calling. He followed his father and older brother to Harrow and then followed them by being chucked out. Unable to get into university due to poor references, he drifted, first working at seismological analysts and then as a gardener before applying to Rada and getting in on the second attempt. Despite one teacher telling him he had "no access to his emotional life", he got his first job while still a student and was soon on the well-worn Fox trail.

It was Kevin Whately who handpicked him for the part that made his name, having spotted him in Colditz. They have just finished filming the seventh and final series of Lewis. He is not sorry to be leaving it behind. "It's lovely. Like a warm bath. But I wouldn't have minded if they'd killed me off – put it that way."

After their theatre stints, he and Piper are moving to LA. He is already honing his American accent, though one suspects the studios might prefer the Old Harrovian approach. "Maybe I'll play up the acting dynasty thing. Be really eccentric and wear a tweed suit..." If it doesn't work out, he has a fallback. He has been recording "quite miserable", Coldplay-style guitar music in his studio for a few years now and has set up his own label, Fox Cub Productions, after turning down a record deal. "Do some covers, release it on Mother's Day… no thanks," he says. "I've made a least £600 from my music career so far. Not bad, is it?" A singing Fox. It's one thing, at least, that sets him apart from the brood.

Our Boys, Duchess Theatre, London WC2 (0844 412 4659; ourboystheplay.com) to 15 December