How We Met: Stephen Baxter & Terry Pratchett

Baxter (left) says: 'We got so involved in ideas there was no one else left at the party but the hostess, staring at us' (Jean Goldsmith)

Stephen Baxter, 54

An award-winning 'hard-science-fiction' writer, Baxter (left in picture) focuses on plausible alternate realities in his work, from Mars mission tale 'Voyage' to climate-change novel 'Flood'. He lives in Northumberland.

I'd read Terry's science-fiction novels from the early 1980s, in particular Strata, which stuck in my mind. It was about an artificial planet, like a sci-fi vision of Discworld, full of jokes and humour, while having something deep to say about the nature of reality. I was never particularly a fan of the fantasy genre, but having read his science-fiction I followed his early Discworld novels, such as The Colour of Magic. It was big, expansive, adventure stuff, full of Dickensian wisdom that will long outlive us all.

We met in 1992 at an Arthur C Clarke event in Minehead: he had on that black fedora hat even then. Over dinner we talked about sci-fi – between us we'd read all the sci-fi written since the 1930s, and that broke the ice. After that we'd go to sci-fi conventions together, while every year our publisher would put on a dinner and stick us together. Terry would say, "So what news of the quantum?" or "What are all these cosmologists banging on about now?" He was really interested in my background of hard science.

Our collaboration, The Long Earth, came about after an evening we spent at our publisher's party last year, when Terry asked me what I thought about an idea he'd had about parallel universes, based around a short story he'd written years ago. We got so involved spinning around his idea that hours passed, taxis came for us and were turned away until there was no one left at the party but the hostess, staring at us – and we got evicted.

We spent months swapping ideas. I'd visit him for long weekends in Wiltshire and we hammered out a rough structure over one weekend. By the end of it we did a totting up; pub lunches consumed: six; hissy fits: three; books still on track: one.

Terry's Alzheimer's [he suffers from a rare form that affects the visual part of his brain] has complicated his work in practical ways: he's had to adapt to using voice-recognition software, he needs the support of his PA in going over drafts, and so on. But what's impressed me most is that he has never let this get between him and the books he needs to write.

Terry Pratchett, 64

Since publishing his first Discworld novel, 'The Colour of Magic', in 1983, Pratchett has penned a further 38 titles in the fantasy series; 'Snuff', his most recent, became the third-fastest-selling novel in the UK. He lives in Wiltshire with his wife.

We met at a sci-fi convention about 20 years ago. Stephen remarked on my hat and I told him, "When you've got no hair they're very useful." We stayed in touch; we haunted the same bookshops and went to the same publishing parties.

What I love about Stephen's books is that they're not so much science-fiction as a reality that didn't quite happen. Take his book Voyage: after the Apollo programme, Nasa wondered about going onwards to Mars by either upgrading its technology or instead taking the space shuttle/space station combo. What he lays out in Voyage was one of the options Nasa had. I liked doing the Discworld series, because I can do lots of things with it, but it isn't a "what if..." [scenario], as Stephen's books are.

I think it does Discworld good if I don't write about it all the time: sometimes you have to get it out of your system. So last year I was looking around for something new and I came across ideas I'd written about [parallel] versions of Earth. They were from before The Colour of Magic was published and I became successful, and the idea languished until last year.

Stephen has hard-science strengths that I don't, so I asked him to collaborate on turning this material into a book. He's done work with other writers, including Arthur C Clarke, so I thought, well, if he was good enough for uncle Arthur he's good enough for me – and if we get the science wrong, he'll get the blame!

So he came to stay at the pub near my house and we started brainstorming. My wife would have pushed me out of the house if he'd stayed, as we spent a lot of time in the house talking during the day – about the book, the professional side of writing and the sci-fi we grew up with.

Yes, we did have a few hissy fits during the writing of it: one initiated by each of us and one shared. I suspect it came down to the fact that we didn't want the other to trespass on our own territory; I didn't want Steve to become the big, "I know how to write real sci-fi" and he didn't want me to say, "Yes, but I've got more fans than you!" Broadly speaking, though, he was sweetness and light. We went through everything each other wrote, though on several occasions I said to Steve, "That's a very good piece you did," and he said, "Actually it was yours, I just polished it up!"

'The Long Earth', by Terry Pratchett and Stephen Baxter, is published by Doubleday on Thursday, priced £18.99