Morse was a mystery. At least, he started out that way. By the end of Inspector Morse's 13- year-run, we had more or less got the hang of why the over-educated detective so rarely got laid. Now, with Lewis, the series' successor, hitting its stride, there is a new mystery for viewers. What's with Hathaway, Inspector Lewis's tall, blond, theologically inclined sidekick? What killed his sex life? “I don't have dates,” he told his superior on Sunday.

What does Hathaway do to his colleagues? Lace their tea with bromide? And what of the actor who so compellingly plays him, 29-year-old Laurence Fox? “He was so buff,” a female colleague mourns, fondly recalling his full-frontal nudity in a thriller called The Hole seven years ago. But in recent years he has played loser after loser in love. In last year's Becoming Jane, a film that rebooted Jane Austen as a Hampshire femme fatale, he played nice but unfanciable Mr Wisley, jilted by Jane. Then in ITV's A Room with a View he was Cecil Vyse, not a name to win the heart of someone with a Bond girl moniker such as Lucy Honeychurch.

So from buff to not so buff, I suggest when we meet in his cottage in rural West Sussex. “Yes! What happened to me? Maybe I've convinced myself that I'm not leading man material. With Becoming Jane they wanted me to be another part who was a bit jokey and buff but I didn't have any interest in it. I quite liked the concept of going, ‘Maybe there is an alternative for everybody. Here is this shy guy who'd like to be able to express himself but can't.' Which is probably more like me anyway.”

He has scrunched his long body up into an armchair. Suddenly he unfolds it. “Actually, bollocks! I speak shit constantly.” Fox - rugby club T-shirt, oversized boots - is one of those public school drop-outs whose speciality is effortless inferiority. He can afford to be self-deprecating about the buff business, for, in real life, he got the girl. On New Year's Eve he married Billie Piper - the nation's Belle de Jour, Dr Who's Rose, and Chris Evans's ex. Crudely tattooed on his left wrist is “Mrs Fox 31-12-07”, a memento of their honeymoon in Mexico. “Drunken moment in Playa del Carmen. And she's got ‘Mr Fox'. But don't tell the agent.”

Just as Evans invited Piper to his latest wedding last August, she invited him to hers. “He's a really nice guy, lovely and they're good mates as well,” Fox says sportingly. “I think there's no point in being with someone if you're going to be proscriptive about why and how you'll be with them. I can't turn round and say, ‘These are the rules.' Actually. I do occasionally turn round and say, ‘I don't like using short knives. I only like using the long ones.' That's about as proscriptive as I get.”

Did he invite Martha Swann, with whom he had been since stage school? “No, I didn't invite my ex-girlfriend, I have to say. We weren't lucky enough to be as good mates as those two are. Maybe in years to come we will be.” The papers gossiped he dumped her for Piper after they met in the Christopher Hampton play Treats. “We had already broken up. I think the press had already had it worked out that I just suddenly ditched her. We were broken up. We weren't going out at the time.” They had broken up once before, he adds. Two-stage break-ups are the best for really prolonging the agony, I say. But Fox, for once, does not joke back.

“Well, it's sad - isn't it? - when things don't work out. But if you're going to spend your life not understanding one another that well... You can still marry and be successful and everything, but for me, and probably for her, we both would have felt a little bit disillusioned after a while. So I couldn't really get into it, I'm afraid.” And then he met Billie? “It was like that,” he says, clicking his fingers. “I love the way people go, ‘When you know, you know.' It's so true. You just do. You just go, “Wow, yeah! Yes please, and thank you.' And it's the best. I'm really lucky. We have so much in common that we don't really row. We occasionally shout at one another but we don't really row, not really moods, either.”

As I approached their home, Piper drove away from it. She has starred in enough interviews without taking a bit-part in another. But she phones several times, protectively, since she is the old media hand. She asks where we are doing the photos and when he says the pub, she asks why not the garden. “It's too council,” he says, “council” being his slang for shabby. He calls her “Bill”; she calls him “Lozza”. Every exchange ends in “I love you”.

It is all very different from poor Cecil in Room with a View, a man, Andrew Davies apparently wrote in his screenplay of the E.M.Forster novel, so closeted he had no idea he was gay. I wonder - just wonder, mind - if DS James Hathaway might be gay too.

“I think it's not f***ing interesting whether someone's straight or gay,” Fox says suddenly animated. “Especially when you come from Harrow where everyone is classified. If you like acting, you're gay. Because of all of that I'm not interested in people's sexuality as a topic of conversation at all. I just try and make him as unknowable as possible. He's a real loner. I think he's just one of those guys that doesn't quite fit in anywhere, doesn't feel happy anywhere and needs to be around other people that don't feel happy where they are.”

I have inadvertently hit a nerve and that nerve is not homosexuality but Harrow, the school chosen for him by his father, the actor James Fox. Unfortunately, Laurence hated Harrow and was expelled. “You bully the shit out of each other and then the girls come along and you just sort of go for it.” He left after a sixth form ball. He is unable to remember the sordid detail.

Afterwards he worked as a gardener in Wimbledon, where he was still living with his parents, and then in an office analysing seismological data. “All I did was smoke loads of weed and read The Sun.” One breakfast, his father asked if he had thought of acting. He tried for Rada, failed to get in and, his gander up, became determined to try again. He asked his old drama teacher from school for help and got in the second time with a recital from Love's Labour's Lost and another from Jim Cartwright's Road - “that was the last time I ever did an accent”.

By the time he left, he had already made his professional debut in The Hole. Its star was Thora Birch, fresh from American Beauty but it was another of the young actresses who impressed the director, Nick Hamm. To his slight irritation, he told Fox she was going to be a star. It was Keira Knightley. “She was really clever. That's what I remember about her and that I had to sort of slightly molest her at some point in the film and feeling really uncomfortable about doing it, because she was 15 and I was 23. The director asked me if I was gay and was that the problem, and I was like no, that's not the problem: I just feel a bit like I'm molesting one of my little brother's friends.”

On paper, Knightley, the actors' daughter from Surrey, looks the sort of gel Fox might have ended up with. Instead he chose a pop star from Swindon who felt, as she says in her memoirs, “as common as muck” at her London stage school. When I met her, I tried to make something of their different backgrounds, but I now see they are well suited: funny, clever, their work ethic checked by a slacker tendency. Their cottage is more Withnail and I than Laura Ashley and I bet they drink and smoke a lot between jobs. In fact I know they do. There is champagne left over from the wedding in the porch. “Dangerous,” he says. “You go, ‘Do you fancy a glass of champagne with your lunch?' And suddenly you've drunk a bottle and you're wrecked.” He gives me a bottle when I leave. So that's me bought.

He thinks his father has a minor crush on Billie. “They really like her. My family have always been, if you make an effort with them they'll make an effort with you. I think if Bill had not made an effort with my family then I think they'd probably think that we were crazy.” He has not read her extremely frank autobiography nor seen her as Belle de Jour, in Secret Diary of a Call Girl.

“I didn't mind. I should work on being more jealous, I suppose, but out of sight, out of mind. I didn't really have any objection to the show. I think it's great she wants to do different things all the time.” Fox makes no pretence that Lewis breaks new ground. “I said to my agent: ‘If I said I'll do 50 more years of Lewis could I play Nureyev. Like John Thaw did 100 Morses and 50 Kavanaghs and was then allowed to be Goodnight Mr Tom?'”

On set he winds up his co-star, Kevin Whately all the time: “ ‘Great scene, Kev. What an actor you are. I love the double eyebrow raise.' He got pissed off with me twice in three years, both times for actually still talking when they said, ‘Action.' He goes ‘shut the f*** up, Lol.' ”

The dedication of a Cate Blanchett, whom he worked with on Elizabeth, puzzles him. “To me she just seemed like she was in another world all the time. I suppose she was. She was gunning for her Oscar.” Yet he is more serious than he lets on. He is not an evangelical Christian like his parents, but he does carry a Bible in his bag and the number of his favourite psalm, 139 (“Search me, O God and know my heart”) as a tattoo on his right bicep. Fox is not only a gas to meet but, I suspect, an interesting man to get to know.

As for getting to know Hathaway, it seems we are about to in an episode that explains why he left his seminary under a cloud. Fox is not enthusiastic. “I knew it would happen and it's all about everything that we thought it would be about. I suppose they want to big up Hathaway in a way, make him richer. I deliberately and actively try to do as little as possible with it, be as ambiguous with my meanings as possible. I think it was probably best to leave it.” He may be right. But I am glad to get Lozza Fox's back-story straight.

Lewis, 9pm, ITV1, Sunday