There is a bit of a gagging order on David Tennant just now. While he is happy and able to chat about life in general, this and that, an uncharacteristic reticence descends with the mention of a certain doctor. Well, you can understand why the BBC is as twitchy as the Official Secrets Act. It waits years for one more Dr Who to swing by, then two show up in a matter of months. Except, not quite yet. The fact is, Tennant has still to take over the role of the repeatedly re-birthed Dr Who from Christopher Eccleston who, the other month, astounded the television show's production team by announcing his departure from the cast. As a result no-one involved wants to tempt fate by saying anything until the next manifestation, the tenth Dr Who, is in the Tardis, so to speak. Meanwhile Tennant's star continues to dazzle in other directions. Scarcely had he time to divest himself of those risque Casanova ruffles than he plunged into another lover-boy persona in Secret Smile, a two-part thriller for ITV currently in production. But this time his role doesn't quite require the same frolicking desire to please which brought such brio to his lithe Venetian Lothario. "I suppose you might describe this latest character as the kind of boyfriend you don't really want to have," he says, which is as good a trailer line as any. And before he can elaborate, the call goes out for Tennant to head for the Secret Smile set and resume rehearsal. But our conversation had actually begun with his part in another forthcoming revival from the archives. As if to encourage the return of the neighbourhood bobby, the BBC is bringing back Dixon of Dock Green, this time as a six-episode drama for Radio 4. Tennant plays PC Andy Crawford, the callow police-college graduate and side kick to the affable PC George Dixon (David Calder), who may be long in the tooth but whose intuition often pulls in the baddies faster than the rule book. "Of course, those were different times when the world was perhaps a simpler and gentler place." In fact it's a world before Tennant's time since he was just a toddler when in the early 1970s Jack Warner, the original Dixon, became a reassuring constant in British life, his "Evenin' all" a catch-phrase benediction. "We haven't updated the plots, just adapted them for radio, retaining the light-hearted touch which maybe evokes a community spirit that's been lost. What's more, though, I play Andy as a young Scot newly arrived down south from Paisley, which is great because I rarely get the chance to work in my own accent." Everything about Tennant suggests he thrives on the pell mell. Slight in physique, he describes himself as "a skinny streak of nothing", but there is a quick-silver quality about him, a Puckish merriment one minute, a sort of wistful angst the next and both moods held together by a determination not to take himself, or present media attention, too seriously. In that sense he belongs to the tradition of the self-deprecating Scot but with the old chippiness stripped away. "It's difficult to be objective about Scottishness when you're Scottish but I think there is an inherent irreverence in us." Was it that wee idiosyncracy which once prompted Tennant to remark drolly that the last thing he wanted to do was sound like Sean Connery in his Spanish castle, talking about the SNP? Not that he was trashing Connery, he said, adding a conciliatory: "He's the boss." But in a trade where Scottish actors are hot box office at the moment Tennant is anxious not to sound triumphal: "I'm very wary of proselytising for Scotland." That word, proselytising is a useful clue to his hinterland. As David McDonald he grew up in Renfrewshire, the youngest of three children of the Very Rev Sandy McDonald, former moderator of the General Assembly of the Church of Scotland, and Tennant's only reason for changing his surname was to avoid being confused with another actor. He remains a committed Christian, but could it also be that his father's pulpit oratory inspired in the son an early sensibility towards theatre and performance? "Well, as a child you're never sure where inspiration comes from. But my parents have always been very supportive and generous, even when at the age of three I told them I wanted to act because I was mad about – wait for it – Dr Who." Tennant's earliest memory of his father is of just how busy he was. "But, yes, I'm sure that his preaching had something to do with my insistence on acting. Actually, he was very good at filling me in on Dixon because I guess that in those days, when he'd finished writing the Sunday sermon, he'd sit down, along with the rest of the country, to watch the programme on Saturday nights. It was that kind of show." Being exceptionally busy might well be a family trait. In the past couple of years Tennant has escaped the actor's occupational hazard of resting as the parts have come rolling in. He'll be there in the new Harry Potter movie, Goblet of Fire, as Barty Crouch Junior, and while sprinting from Casanova conquest to conquest across our television screens, he was also on stage in Bath in John Osborne's 1950s classic, Look Back in Anger. There, in Richard Barron's searing production, Tennant portrayed Jimmy Porter as a deeply damaged individual rather than a venomous proponent of class struggle. "I didn't set out to play him as not belonging to the fifties. I simply saw the play as a brilliantly astute piece of writing about the human condition. All you can do as an actor is try to see the character through the prism of your own experience." And prior to Casanova there were two other memorable TV performances, the first as the unctuous, gold-digging parson in the splendid dramatisation of Trollope's He Knew he Was Right. And the second in the surreal musical thriller, Blackpool, where he played the love-lorn detective, Peter Carlisle who dangerously becomes entangled with the wife of his main suspect (Sarah Parish and David Morrissey), with barking consequences all round. "Blackpool was tremendous fun. On the page it all looked quite possible to achieve but when we started rehearsing it was: 'Jings, how are we ever going to make this work?'; Learning dance moves, and singing and acting at the same time, it's not easy unless you're a full-on musical theatre twirlie." Did he really say "jings"? Tennant laughs. "Well, I started using the word ironically but now it seems to be stitched into my vocabulary even though I'm aware it sounds a little quaint." But what influences did he draw on for Trollope's nonsensically craven cleric? "Ah, nothing close to home, for sure. When I got the part I rang my father to say: 'You'll be delighted to know that I'm playing a minister of the church. Unfortunately he's not a very nice minister of the church.'" The make-up team put a wave in Tennant's hair for the role and with a dog-collar on he began to look a little like his father. "Not a bad thing, that. A fine looking man. Still, it takes you aback, seeing yourself turning into your parents." As for Casanova's fun and games, he says his parents saw the series as just that. "And like many others, they seemed to enjoy it a lot." Tennant is 34 and single, a graduate of the Royal Scottish Academy of Music and Drama, whose career took off at the National Theatre and the Royal Shakespeare Company. In London his role in Lobby Hero, at the Donmar Warehouse, gained an Olivier nomination, but he loves this chance to leap from stage to television to film to radio. "It's exhilarating, a real pleasure to dabble in all." But he also knows the sting of repeated rejection. In Scotland Tennant has the distinction of 16 failed auditions for a part in Taggart. "I'm the only Scottish actor alive who hasn't been in Taggart. Some have been in it so many times they've played at least three different murderers. But I wouldn't like to claim any Zen-like ability to deal with rejection. You've just got to shrug off disappointment as part of the job." When Tennant first became a Dr Who aficionado, Tom Baker was in the role. Now, as much as he can say anything about the next series, it's that he sees the prospect of taking on the mythic mantle as both daunting and exciting. "Chris (Eccleston) has done a terrific job in reinventing the character for a modern audience, and he's a very tough act to follow." But Russell T Davies, the writer and executive producer, says that, since regeneration is a huge element in the programme's appeal, today's young viewers will have a complete Dr Who experience when they witness their hero undergo a change of face. Shooting the new series will begin this summer in Cardiff. And when will we see the latest incarnation soaring to the top of the ratings? Suddenly, David Tennant becomes cryptic: "It's all still to happen," he says. "So, who knows?" Who knows, indeed. Dixon of Dock Green begins on BBC Radio 4 on June 15 at 11.30am.

David Tennant: the story so far Age: 34. Childhood: Renfrewshire. Education: Paisley Grammar School; Royal Scottish Academy of Music and Drama. Career: First break in the Scottish TV series, Taking over the Asylum.. Highs and lows: "Acting is a bi-polar existence of highs and lows. I'm just very lucky that each thing I've done recently feels like a high." David Tennant won the title of best actor in the prestigious Critics Awards for Theatre in Scotland (CATS) yesterday. He took a break in his busy filming schedule to return home to lift the title at an awards ceremony at the Tolbooth in Stirling. He beat off competition from Nabil Shaban, ironically a former Dr Who villain, to win the prize for his performance as Jimmy Porter in Osborne's Look Back in Anger at the Royal Lyceum Theatre in Edinburgh. He was presented with the award by Scottish National Theatre director Vicky Featherstone. The overall winner at the ceremony was a play called The Wonderful World of Dissocia, which took five out of the 10 available awards – for best new play, best director for Anthony Neilson, best actress for Christina Entwisle, best design and best production. Co-produced by The Drum in Plymouth and the Edinburgh International Festival, in association with Glasgow's Tron Theatre, the best production award was collected by festival director Sir Brian McMaster. He said: "The Edinburgh International Festival aims to bring artists and organisations together to create new, challenging work that audiences might not otherwise have the opportunity to see. "This co-production brought a great team together, with fantastic results." The 2005 CATS awards were open to any professional theatre work produced in Scotland between May 1 last year and April 29 this year. Some 168 productions were considered for this year's awards.