@glasgowalliance, damn, I was in Balloch last month, I could have nipped down for a coffee !!

When Christopher Eccleston grins at you, it is hard to know whether to smile back at him, or to jump on a chair and scream. It is the eyes. Hypnotic, glittery things that make you ponder two questions: is this a nice man - or is he about to go for my neck?

You may feel the same when you tune into the new series of Doctor Who on BBC1 on Saturday and watch the scene in which he first meets his new companion, played by Billie Piper.

She is in the basement of a London department store, being pursued by an army of shop-window dummies. Just when it looks as if she's going to be dismembered, Eccleston appears behind her, introduces himself, and urges her to run for her life. You wonder which direction she ought to flee - away from the plastic monsters in neatly ironed leisurewear, or away from this leering, goggle-eyed, jugeared bloke in a leather trench coat.

"I was told that some executive had shown the programme to his children," says Eccleston, "and they asked, 'Is the Doctor good or is he bad?' I was pleased about that. The Doctor is brutal at times, he's confrontational, inflexible. He sometimes creates carnage. There's nobody like him in Disney."

So will we trust him? Eccleston looses a dirty cackle. "I wouldn't," he admits.

Doctor Who faded from our screens in 1989. In 1996, there was talk of reinventing the programme as an expensive transatlantic coproduction. But all viewers got from that deal was a 90-minute TV movie starring Paul McGann. Now the Tardis is ready to materialise again.

Eccleston says he was puzzled when he heard that Russell T Davies - the man behind Queer as Folk and Casanova - was overseeing the Doctor's return. Why would a man widely considered to be the country's finest (and most employable) television dramatist be bothering to revive a knackered old sci-fi series?

"But then the perverse side of the whole idea began to appeal to me," he says. "So, I sent Russell an email and asked for an audition." Once Davies had received a polite refusal from Hugh Grant, Eccleston's offer was accepted and the path was open for a Doctor aggressively different from his predecessors.

This incarnation of the Time Lord from the planet Gallifrey is no frock-coated Edwardian gentleman. He's brave and wise and brilliant, of course - but he's also rough and ready, down-to-earth, and talks with Eccleston's own Salford accent. "I wanted to show," the actor says, "that you could be clever and authoritative without having to speak in RP [received pronunciation]."

That goes for his assistant, too - Billie Piper's Rose Tyler is a south-London shopgirl with no A-levels, who dresses like a member of Girls Aloud. And, in keeping with this sense of realism, the new Tardis crew won't venture too far into the depths of outer space: they're more at home on a Kennington council estate or in a shabby British suburb.

"It's always brought back to Earth," Eccleston says. "It's not about fighting aliens on the planet Zog. It's about real people." His Doctor, he asserts, will have an emotional life much deeper that most sci-fi heroes.

TV's most charismatic character is, he adds, "a car crash between me and Russell T Davies. I looked at Russell's verbal dexterity and energy and his speed of thought, and said to myself, 'I'll have that, thank you very much.' And Russell is such a committed Doctor Who fan that the writing has probably ensured that I'm playing a version of Patrick Troughton or Tom Baker or William Hartnell.

"But it's also a version of me as a kid. The Doctor has that insatiable curiosity that I was lucky enough to have when I was young."

It is a young audience that the actor has in mind. "In my heart I'm thinking of the eight- to 12-year-olds. I think if you make very good, demanding, intelligent television for children, the adults will come. But I'm doing it for the kids."

It may, he suspects, be a hopelessly idealistic aspiration, but he'd like the new Doctor Who to be watched by the entire family - just as it was in its heyday. The BBC, which has scheduled the show against Ant and Dec, is dreaming the same dream.

Of all the actors to stand at the helm of the Tardis, Eccleston, 41, is probably the one with the biggest pre-Doctor Who career. The TV credits on his CV are prestigious: Cracker, Our Friends in the North, Hillsborough, and Russell T Davies's The Second Coming.

On the big screen, he breathed bitter life into the hero of Michael Winterbottom's Jude, lent a manic nastiness to the deranged military commander in Danny Boyle's 28 Days Later, and brooded away opposite Cate Blanchett in Elizabeth.

He also made the trip to Hollywood to play the villain of Gone in 60 Seconds - an only-forthe-money gig in which he believes his performance was as terrible as the movie. ("I'm a bit slapdash when it comes to choosing films," he concedes. "With television I'm much choosier.")

But there is something that most of these characters have in common. They are troubled, tortured types. And Doctor Who, Eccleston says, has given him a chance to show that he can play a character who is rather more larky.

"I'm not known for my charm or my comedy. But you should try to do the things you're not good at, so you can learn a lesson. Whether it'll be a costly lesson, I don't know."

He will soon enough. Doctor Who is a job for life. Even if the new version of the show is a disaster, Eccleston will always be in demand for conventions and spin-off productions. (Most of the old Doctors earn a bit of holiday money from a monthly series of audio-only adventures on BBClicensed CD.)

Oddly, however, Eccleston professes not to have given much consideration to the effect that Doctor Who will have upon his everyday existence. Perhaps he suspects that the presence of tabloid-magnet Billie Piper in the cast will ensure that Heat magazine keeps out of his face. (On the first day of filming for the new series, the long-lensers were out in force to capture shots of Eccleston, Piper and a dwarf actor dressed as a pig in a space-suit.)

"Maybe it's naive of me, but I'll deal with it as it comes," Eccleston sighs. "I've done quite a lot of work up to this point, and I intend to keep that variety going if I can. But, so far, I'm very glad to be part of the world of Doctor Who.

"I work in quite a cynical industry, and there is something at the heart of this show that's very pure and very innocent. I've been quite touched by its message: Isn't life extraordinary? Let's relish it." And he grins his alarming grin.

Maybe we won't trust this new Doctor - but when he asks us to step aboard with him on Saturday, the invitation should prove impossible to resist.