Anne Enright writes about the difficulty of starting a novel, and the tricky conditions required for doing so, including one's own emotional relationship to the material. What comes over is an impression that the ability to get going on a piece is not ultimately under the author's conscious control or will.
I've written something similar on this blog, but after reading the Enright piece I got to thinking that actually, sometimes being forced to start writing rather than waiting around for the ripe moment or 'inspiration' is useful, and fruitful (well I hope it was for me!). All but the first two of my radio plays were written to commission and deadline, as were at least two of the stories in Balancing: 'The Way to Behave', which was commissioned for the Bitchlit anthology, and 'Into the Night' which was requested for a Welsh anthology of erotic stories (though the anthology never happened - not enough Welsh writers came up with erotic stories!) I've also written here agreeing with AL Kennedy that literary competitions which dictate subject-matter militate against innovation, but both of these short stories were responses to prescribed themes.
I guess, though, it's more a question of being lucky if the prescribed theme or the imposed deadline fits your own prior state of readiness, because unless you're in the 'zone' whatever you write will be dead in the water before it swims.