Time for a short story.

"In general throughout my life I've had these little moments where, if something had gone a tiny bit differently, I would be dead," Lionel Shriver tells me. She is in America. Fall is coming. The end of summer, she says, always makes her sad.

"And I had reflected," she continues, "that this was probably a universal experience for grown-ups. This little trove of memories. And they're very private, they're often something you don't even talk about because they're about something that didn't happen. They're not even stories."

Except they are, of course. She even tells me one, about the time she went scuba diving and almost drowned. "Someone saved my life because I panicked. And you cannot hold your breath and rise quickly. It will kill you."

This is her story then, but not just hers. It reveals the truth for all of us, the truth that "sometimes it doesn't go your way. Some day it's not going to go your way. And that is the reason those moments are so powerful. Because it's a crude recognition of mortality and vulnerability which always comes as a shock because we do walk the world as if we're going to be here forever."

This is the thing of it. At any moment we could die. We could step off a kerb at the wrong time. We could be walking along thinking about dinner and something could fall from the sky on top of us. We could go swimming and get caught in the current.

Some time ago Shriver read a little news story in a New York paper about a young woman, "widely liked", who fell off the balcony of her building. She thought about that, thought about her own near-death moments, and between them something clicked. And because she's a writer, she wrote a story.

The result was Kilifi Creek. In it an American girl goes to Africa, imposes herself on people she hardly knows, goes for a swim in a nearby creek and gets into trouble. "Young people are more fearless, right? And more likely to get themselves into scrapes as a consequence because they have such untested faith in themselves. Which is appealing."

It's a simple story about the possibility of death but it's also about Africa, landscape, youthful arrogance, middle-aged irritations and fears, chance and misfortune. It's less than 6000 words long but - like Kandor, the miniature city in a bottle in the Superman comics - it contains multitudes.

Originally published in the New Yorker, it is now in contention for the BBC National Short Story Award. Shriver will be over in the UK for the prize-giving.

"I will be back in time to lose in person," she says, with a laugh. In conversation this is how she speaks; direct but with a desert-dry humour.

Shriver sees herself as a novelist first and foremost. Readers do too, more than likely. Her name will conjure up memories of We Need To Talk About Kevin, that chill, dark book about parental lack of love and American high school massacres. But she's been nominated twice before for the National Story Award ("I lost both times," she points out, but that won't stop her getting her hopes up) and she enjoys the form. "One of the things that's exhilarating about short stories is what you can leave out. I sometimes get tired of the burden of all my characters in a novel having to have parents and be from somewhere. I'm not always interested, and in the short story there's no obligation. That's just liberating. It's a mutual liberation because the reader's not necessarily interested in every character's parents either."

It's easy to tell which ideas are suitable for novels and which for something shorter, she believes. "It's just a feeling of scale and whether you could possibly achieve it in a few pages or whether you need hundreds of pages. A novel idea tends to branch out whereas the short story has a kind of simple gist to it. That doesn't mean unprofound, but it has to be something you can encapsulate quickly. And I think the art of the short story is being able to give it that kind of density of meaning and yet still have the tale itself unspool with at least the illusion of effortlessness. It should be inviting. I think all fiction should ultimately be easy to read."

Is the writing process very different for writing a short story? "It's faster," she points out with amusement. "A lot faster." To read too, of course. Short stories can't hang about. "You can't put the main story off too long, which is probably a good thing for me because I am compulsively digressive. I am 90,000 words into my new novel and I still haven't got around to the main event."

Plus, if the story fails, if you can't make it work, that's okay. If a novel fails, that's different. "If I have to throw out a whole book, at least in terms of my own tiny little life, that's a tragedy."

Most people prefer immersing themselves in novels, and Shriver does too, but that doesn't mean the short story is a less substantial form. "If you're reading the right people then the short story can deliver a surprising amount of punch for the amount of time you give it. William Trevor, for example. I actually think he's a better short story writer than he is a novelist. I get as much out of a single short story of his as I do out of whole novels."

Push her for other short story writers she admires and she names Jess Walter's recent book We Live In Water. "It's quite fresh, it feels modern and spare. It's got a sly sense of humour and there's an airiness to those stories, which is not to call them lightweight but they have that ease I was talking about that invites you in right away. And you know, I like Alice Munro, but that doesn't make me special."

Writers are readers too then. And readers? Well, as Shriver says, "we tell stories all the time. They are effectively short stories. If you have a long relationship with somebody you can ultimately convey whole novels. But, for the most part, in our interactions with each other we are dealing with compressed narrative. And all of us - especially if we are socially successful - are good short storytellers."

This story ends here.

Lionel Shriver's story Kilifi Creek has been shortlisted for the BBC National Short Story Award 2014. The winner will be announced on September 30 live on BBC Radio 4's Front Row at 7.15pm