Since he was just three years old, David Tennant has passionately wanted the role of Doctor Who. No doubt he wasn’t the only one who thought he could play a Time Lord – but it was an unlikely aspiration for a boy who, like Gordon Brown, was a son of the manse.

And unlike all those other little boys, he did indeed grow up to play the Doctor – currently in his 10th incarnation on Saturday night television.

His father, the Rev Sandy McDonald, is a Church of Scotland minister in Paisley, near Glasgow. So David went to church regularly all through his childhood – and his father also happened to be the minister for his primary school.

According to David, Sandy is “a dramatic, lively, engaging and entertaining speaker” (and I can attest to this, having seen him in the pulpit).

Was it witnessing the effect and popularity of his father’s addresses, I ask, that drove him towards acting as a career?

“Possibly. I was very young when I decided that’s what I wanted to do, so it’s hard to know how much of a conscious influence Dad was.

“I remember, after seeing Jon Pertwee turn into Tom Baker in Doctor Who, having a conversation with my parents at a very young age about actors and what they did. I remember getting the distinction between a character and an actor, as they explained it. I understood what fiction was very clearly – and I always feel uneasy when people talk about children not understanding the difference between fantasy and reality.

“I can only have been three, and was just enthralled by [Doctor Who]. But I was quite clear that I didn’t want to be a Time Lord – I wanted to be the person who played a Time Lord,” says David, who had to change his surname to Tennant when he started acting because there was already a David McDonald in Equity, the actors’ union.

His parents, though always encouraging and supportive, tried to steer him towards more secure careers.

However, David tells me that he “never once wavered” from his chosen path – rather extraordinary, considering that he’d decided on his future while still a preprimary school toddler. Steeliness is probably his defining characteristic – and he was obviously just as steely then as he is now.

We know each other well, having first met in October 1993. As a jobbing actor, I’d gone to Glasgow to play a part in a series called Takin’ Over the Asylum – about a hospital radio station based in a mental health facility. My first scene was with a young Scottish actor – it was of course David, who was playing a bipolar teenager.

He was 22 and had only just started acting, yet his confidence and determination were extraordinary. We hit it off straightaway, and early the following year he moved to London where he rented a room in my house for five years.

His subsequent success owes much to his aforementioned steeliness. That is not to say he’s ruthless, unkind or lacking in generosity; quite the contrary. But he is absolutely determined – in fact enviably so for a man in a business that positively engenders insecurity and self-doubt.

He has always been able to look after himself. At 17 he left home to go to live in Glasgow – first with his older sister and then with a succession of fellow students – until he graduated from drama school in 1991. This made him unusually self-sufficient for his years.

And he’s very organised. When we lived together I was always teasing him about his alphabetised CDs, for example.

Still single (the tabloids have, usually erroneously, linked him with various women, including Kylie Minogue), David is pretty careful in all his choices. The only area in which he goes positively mad is in his choice of clothes. In fact his wardrobe can be very flamboyant, which is why those who know him quickly gave him the monicker of “metrosexual”.

In the early days many of my friends (principally male, I’ll admit), thought that he must be gay. “He has to be – you’re his best friend, and look at the way he dresses,” they’d protest.

Leaving aside the suggestion that an association with me reflects on a man’s sexuality, I had to break it to them that just because a guy wears a red velvet suit and is able to form a close friendship with a woman he isn’t sleeping with doesn’t necessarily mean he’s homosexual.

David, meanwhile, took all this teasing in his stride; he is so unmacho and fair-minded that the speculation about his sexuality never bothered him . “Why would it?” he’d say. Now, that’s what I call a real metrosexual.

In all the time I’ve known David he’s never been out of work. It’s not that he is the best actor in the world – it’s more that his success is down to a fusion of talent and unshakable self-belief.

It’s quite bizarre witnessing how famous he’s become. Often, when we go out together, large groups of women will visibly go weak at the knees. Does he mind being pestered by fans?

“It comes with the territory,” he says. “Some days it’s tricky; other days, not at all – and, anyway, who am I to complain about how hard it is to be famous? It’s not.”

Unlike some, David doesn’t leap indiscriminately at the endless party invitations and goodies offered to celebrities; he’s also increasingly reluctant to give print interviews.

Is this because he’s a control freak, I suggest, and doesn’t like the idea of not being able to control what’s written about him?

“Erm, it is slightly controlling of me, I agree,” he says. “But it’s all too easy to become defined by your press cuttings. I’m much happier going on a radio show and talking nonsense for 20 minutes. I’m an actor, after all.”

He also subscribes to the Marlon Brando take on actors who sound off about world affairs: “Just because a guy’s famous doesn’t make what he thinks interesting.”

But, I point out, I know he cares about climate change – and he did just buy a hybrid car.

“Exactly,” he replies. “Now, I’m clearly no Al Gore, but I liked the idea of doing my bit. And then you get someone telling you that the environmental cost in manufacturing a [Toyota] Prius [petrol-electric car] is worse than driving some gas-guzzling monster for 10 years.

“I have no idea what the truth is, although of course I should have found out. The person who told me that may have a grudge against Toyota. So I really don’t want to get into the trap of sounding off about anything I don’t know enough about. It’s very easy to get seduced into believing your opinion is informed just because people ask you for it.”

What about using celebrity to promote a worthwhile cause – isn’t that a good idea?

“It is a fact that sprinkling a bit of celebrity over certain causes probably can make a difference. And maybe it’s pompous to sit at home thinking that I want to preserve my integrity when, actually, if you waved a wee flag for something, it might make a few quid for a worthy cause.

“It’s a balancing act that I find a bit of a struggle, to be honest. Mostly I just muddle through, trying to do the decent thing from moment to moment.

“Is it churlish to have the power to influence things and choose not to because you fear being misrepresented? I don’t know. I don’t manage to be consistent about it, anyway. I’m never sure how vain to be.”

It takes a cool head not to be inflated by the news that fans voted him the best Doctor Who of all time, in a BBC magazine survey. And they will certainly be pleased to know that David hasn’t come close to tiring of his Time Lord role yet. Indeed there are three feature-length specials on the horizon.

Meanwhile, he is about to take on the arguably more challenging role of Hamlet for the Royal Shakespeare Company. Naturally, he’s quite nervous about scaling this particular rock-face – but then, if he weren’t, he’d probably cock it up. “I always have that Presbyterian voice in my head, saying, ‘Could do better’,” he admits. “But I quite like that.”

The new series of Doctor Who continues on Saturdays on BBC1

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010