Was it the lighthouse keeper shimmering with an alien light?

Or the robot eyes ablaze with the urge to kill? Or the pitted brain quivering in a pool of sickening gloop? Whatever it was, there is usually one moment - one wonderful, scary moment - that every fan can point to as the one when they fell in love with Doctor Who.

The writers of a new collection of Doctor Who short stories aren't any different. They all remember their own version of that moment - when the series became entwined in their consciousness.

Except now they have the chance to go one better; to create their own version, their own moment. Somewhere out there children are just waiting to be scared.

The new collection, Doctor Who Short Trips: How The Doctor Changed My Life, was the result of a competition to find writers new to Who. More than 1000 people pitched stories to the editors with 25 being chosen for the book: the winner, by Michael Coen from Glasgow, and 24 runners-up. The result is 25 new exciting little trips sideways, backwards and forward in to the world of Doctor Who.

The fact that the stories are all inspired by the same programme yet are all so different proves just how bendy the format created in 1963 still is.

There are stories in the future, the past, the present, here, there, now and then, yet they are all still Doctor Who. It's one of the reasons the series has lasted as long as it has, but it does make it hard to pin down what makes a Doctor Who story work.

Simon Guerrier, who edited the new book and is the author of several Who novels, says diversity is one of the keys - and there are a whole big jangly set of keys - to the success of the programme and its spin-offs.

"Doctor Who is a great story generator," he says. "The Tardis can land in different types of story - a western or a murder story or whatever."

Michael Coen agrees: "Doctor Who has the best-ever premise for generating stories - a man steps out of a magic box into an adventure."

So diversity is one of the factors in good Doctor Who. But what else? Why does a show created in 1963 still work in 2008 with only a few tweaks here and there by Russell T Davies, the six-foot-something Welsh Who saviour man.

Well, there's Fear, obviously. Terror has to be on that keyring of success.

Robert Holmes, who wrote some classics for the series in its Tom Baker heyday, was reported to have clasped his hands together in an editorial meeting and said: "Right, let's scare the little buggers to death." Thirty years later, Stephen Greenhorn backs up the idea that fear is important to good Doctor Who. He has written two episodes and in one of them, the brief was simple: terrify 'em.

I had to scare the living daylights out of kids . . . it's about haunting them and keeping them awake at night

"One thing I had to do was scare the living daylights out of the kids," says Stephen. "It's about not just engaging children but haunting them and keeping them awake at night." But don't worry, mums. This is good fear; healthy fear.

And after fear, there's arguably the most important key of all: the Doctor himself. The genius who saves the universe. The open-eyed, open-minded wanderer.

For Terrance Dicks, that is what the programme is all about. Terrance Dicks, in case you don't know, is the most legendary Doctor Who writer of them all. If this was pop, he'd be McCartney. He has written 70 novels, stage plays, telly episodes and spin-offs.

It was Terrance's books that made me love Doctor Who, made me write stories of my own, made me keep going until my own story was published in the new book.

When you sit down to write a Who story, you appreciate how clever and complex and brilliant the show is, but Terrance was writing before DVD. Before video. Before regular repeats, even. This was a time when Terrance's books were video recordings of the mind, the only way to experience the show again.

He obviously still loves the series and when we talk he's in a giggly mood, loving the reminiscing and geeky analysis. And he can't say it often enough: it's all about the Doctor. "The Doctor is always the Doctor," he says. "If he's old and white-haired, large and burly, distinguished or whatever, it's always the Doctor and it's important to have a grip of that. There is Doctorishness. There are Doctorish lines or Doctorish reactions. It's the Doctor's show and it's less interesting when he is not on the screen."

But maybe this talk about reasons and storytelling and character and keys is missing the point. That's what Terrance thinks anyway.

I let the phrase "story arcs"

slip into the conversation at one point and he pounces. "Story arcs my arse!" he lets rip with a wonderfully throaty laugh. "I don't know what a story arc is!" Terrance prefers "lines". "It's like a set of parallel lines. You stay within those and you've got enormous liberty."

Paul Cornell, who has written for the series and spin-off novels, also has a hard time with the idea that Who is whatever you want it to be. "It's not a free form at all," he says. "When you're reading and watching Who, you may not be able to pin down what the format is, but you know when you see what it isn't. It's an attitude, there's an open-mindedness to the cosmos; the Doctor finding non-violent solutions to problems."

The Doctor doesn't shoot and ask questions later; he just asks questions. "The Doctor is a wonderful creation," says Michael Coen. "He stands up for what he believes in by asking questions rather than by using force."

Of course there's no denying the nature of Doctor Who storytelling has changed a bit, midwifed by Russell T Davies. Thanks to him there is a bit of gulp emotion in the show now. Rose and kissing and stuff. But somehow, Russell has managed to move the ornaments on the mantelpiece to make it look better. And it's still the same thing. The same show.

Paul Cornell says that extra emotional sheen was vital for any series in 2008. "You can't have an effective story without emotional content," he says. Then again, Paul and I secretly admit between ourselves that we've always liked the fact that the Doctor isn't doing an Oprah on us, isn't gushing out emotion, that the Doctor is a different kind of hero, a geeky scientist who glories in knowledge. And even wears glasses. "It used to be the show for the bullied child," says Paul. And I bet you it still is.

Talking of children, is Doctor Who for kids? That's the last question. The last key. "It's clearly aimed at kids," says Guerrier. "It's bright and sassy but what's great about it as a kid is it doesn't occur to you that it's a kids' show. It is the most serious mad adult show, terrifying and strange."

There's something wonderful about what happens when children love something and embrace it.

One of the joys now Doctor Who is back on the telly is that children adore it again; that you can walk into a shop and see a boy - or a girl these days - hugging an inflatable Dalek. "The first time I saw that in 2005," says Simon, "it brought a big smile to my face."

So, it's all about providing enough for the children (and the grown-ups), a dollop of fear, Doctorishness, diversity. Those are keys to good Doctor Who. But not all of them. There is another one just beyond the tip of my fingers, just past the reach of my words, intangible. And the thing is: maybe it's the one that really matters.

Doctor Who Short Trips: How The Doctor Changed My Life is published by Big Finish at £14.99.

Big Finish website