Trawling through my chaotic file system, I came across this, which I wrote a while back – it has at least the merit of brevity (‘Brevity is the soul of wit – if you can’t be witty, at least be brief’) :

‘A character must do, not what I want him or need him to do (for the sake of the story), but rather what he would do in the circumstances – so, the author seeks to contrive the circumstances from which the story will arise, as it were naturally.’

Hmmm. Might just be something in that.