Margaret Atwood: It may seem strange that I'm co-writing a serial zombie novel with Naomi Alderman, being posted chapter by chapter on the global story-sharing site, Wattpad. Why Wattpad? Why Naomi? Why zombies?

First, there's a lot of angst from publishers and writers about the young "not reading", which may mean they aren't reading what older people want them to read. My college teacher, Northrop Frye, said you should let the young begin with whatever interests them; and millions of them are interested in Wattpad. Why Naomi? She's an accomplished novelist and also my Rolex Mentee, and we're exploring many facets of writing and publishing together.

But in her other life – and, as a self-supporting writer for 40 years, I understand the necessity of second lives – she's a videogame author, most recently of Zombies Run!, which has seen her and her co-creators funded to develop the prototype for a similar exercise aid by the NHS. So it's a good fit; and the meaning of the zombie obsession intrigues us both. Naomi has written about it in Granta, I recently made a speech about it at Princeton University. Don't dismiss the zombies: they have Deeper Meaning. They just don't know it.

Naomi Alderman: The zombies definitely have a deeper meaning – anything that intrigues and fascinates people has something to tell us about ourselves. So why do I feel defensive about being interested in zombies as well as the evils of fundamentalist religion? About researching mythical monsters as well as the history of the Congo? The hiving off of "fantasy" in bookshops is, and always has been, a nonsense. After all, who wrote about witches, monsters and spells? Homer. Who wrote about fairies, wizards and ghosts? Shakespeare. Zombies can be as serious as any Caliban or Poseidon.

But our zombies aren't really serious. That's the pleasure of them. Part of the delight of working on this with Margaret, whose work has influenced and inspired me all my writing life, has been to see that, yes, even serious writers don't have to be serious all the time. From the first lines Margaret wrote, this has been a black comedy.

We're still writing the end of the story. I'm holding on to my last chapter because I don't want this to end; it's been an exhilarating, hilarious, joyful process. Waiting to get Margaret's chapter, reading it gleefully, trying to come up with something that might take the story in an unexpected direction or give her a problem to solve. Then waiting again to see what she's come up with for me. Remembering that writing is storytelling, and storytelling, after all, is play.