So, you've written a good deal of longish stuff, and know something of what it takes to sustain a project. And you've got lots of ideas for stories, and several of them look promising for a book-length project. The interactions and conflicts they set up might be enough to fuel a novel, or the seam of travel or life that you're drawing on is rich enough for your creative non-fiction. But of those promising ones, which should you commit to?
How can you make sure that, some months of research and writing down the line, you won't realise that this was the wrong project? How can you be sure that when you reach the (almost) inevitable Thirty Thousand Doldrums, you'll have the stamina to haul yourself across and out of them? And how can you decide that this project is one which, further still into the relationship, you'll still love enought to resist the lure of The Other Novel, and stay faithful to the end?
You can't, of course. And even if, a year or three from now, you look back and think you made the wrong decision, you'll never be sure. So, in default of an answer, here are some questions to ask yourself, which might help you to decide:
1) Which idea looks likely to make a rich experience for the reader? Which will provide you with the opportunity to build in the most drama, big changes, compelling voices, settings which are gorgeous or grim? Of course, it depends what kind of writer you are whether the best opportunities are those to be found in space, in Troy, or in the back yard of your local pub.
2) Which is going to land you with the most heavyweight research? Will that be a bad thing (work, time, money)? Or a good thing, in forcing you you find ways to turn Rose Tremain's "inert data" into living prose?
3) Which idea is most likely to exploit your strengths as a writer, whatever you feel they are? If you're not sure what they are, ask your close writing friends - you might be surprised. Which idea will enable you to avoid the things you're not so good at? For a rumination on that issue, click through to Ask Your Talent.
4) On the other hand, which idea is most likely to stretch you as a writer? Are you willing to take the risk that this first shot at a thriller, or a spaceship, may not go so well? On the other hand, the challenge might draw writing out of you that you never imagined you were capable of.
5) Which world will you most be happy to spend time in? Happy, that is, in the endlessly-intrigued-and-curious sort of way. That might not be the most obviously appealing world at all.
6) Which idea embodies the theme and/or situation which is most potent for you? Not just interesting and potentially dynamic - all sorts of ideas can be that - but something which will, as it were, raise your writerly heart rate and blood-pressure? I'm not normally one for the tortured-artist school of writing, but I do think that our best work is rooted in whatever situations and themes are most potent for us: Jerusha Cowless explored that here.
7) What idea is most likely to find an audience? Not necessarily most likely sell, because that's only one kind of audience-finding that writers do. But it is worth thinking not just about what elements, themes and conflicts intrigue you as a human and a writer, but what are most likely to intrigue readers as humans and readers. If the idea you want to write for all the other reasons, fails on this reason alone, then ask yourself: how will you make this outwardly unpromising idea intriguing, and then irresistibly compelling?
When you've got tentative answers to some of these questions, then it's only sensible to commit a bit of time to testing the idea: reading round the topic, thinking about where the drama in the story would come from (I've tried and abandoned dozens of ideas because the initial idea was interesting, but I couldn't find the story that would power it). You could try freewriting, brainstorming, writing a single scene which at the moment you think might be at the heart of the conflict, or a trivial scene which nonetheless will get your character-in-action acting most characterfully. You could sketch out a plot, or a story, or do whatever other plan-y things you like to do, just to see how it feels. No commitment, no bones broken. How does this feel? Like a house you could live in? Like a job you couldn't just do, but really enjoy throwing yourself into? Like a (wo)man you could marry?
And there's one much more intuitive, a-rational test to help you choose between the possible ideas: which is the project and the character that won't go away? Which do you keep ignoring but it keeps on clamouring? Semi-ignoring it, while you do other projects, is a good test, partly because it never does a project harm to sit on the back of the stove, gently simmering and having things added to it while it stews down. But it's also because of the value of forgetting: using your memory as a sieve may be the best way to choose a project. Sometimes the one you end up writing should be the one that just never went away.