I think questions can be immensely helpful - it's Richard Sennett's thing about the key to craftsmanship being problem solving: once you've worked out what it is that you don't yet know, you're a long way towards being able to make it all work. The two questions I most often ask myself are: "Where do I need to be by the end of this scene?" and "What does she want/need? (and are they the same or not?)".
AdaB, thinking about your question of plotting from the other thread, I think it can clarify things to separate out the writing from the imagining. A lot of creative-writing practice assumes that the writing is part of the process of finding out what the story is - what we're trying to say - what these places and these people are like.
And it's true, of course. Like everyone else on WW, I've got things in my work which just, literally, came into my head as I was writing the scene, and turned out to be hugely important in terms of theme or plot or character. If you ever use freewriting do develop such things - or just as a keeping-the-engine-oiled daily (yearly?
) practice, you'll know what I mean.
But it's horribly easy, if you don't know where you're trying to get to by the end of this scene, as part of where you're trying to get to by the end of the story, to write reams of well-imagined, well-written stuff which reads perfectly well, and does sod all to advance the story with the sort of energy which is needed to keep the reader reading.
On the other hand, it's asking an awful lot of your creative imagination to simultaneously do so many different, multiple, high-resolution kinds of imagining: the big moves of the plot (where you're trying to get to), the finer details of who's doing what and who knows what at this stage of the story, and the close-up details of choreography, character-in-action, sensory evocation, dialogue, voice, etc. etc...
(I blogged about this here, which might be interesting, not because you're trying to write with ruthless efficiency, but because it casts an interesting light on this separating-out of the imaginative process, and the writing-the-actual-words-to-tell-the-story process:
http://emmadarwin.typepad.com/thisitchofwriting/2011/12/dreaming-the-map-the-efficiency-of-magic.html )
A propos finding yourself needing to wedge Scene D in between B and C, I think that's a function of trying to write your first novel, frankly: it's a huge, huge learning curve, and in the long term even every writer who reckons they've more-or-less worked out how it suits them to work, has a different way of coping.
Don't forget that realising that you need Scene D doesn't mean you have to drop everything and write it. You might just need to make a note with whatever's occurred to you so far: "Scene needed here: she sees a sweetshop and gets her idea for her new business, and sees his reflection in the window talking to a glamorous woman who she assumes (wrongly) is his wife"
You might find this other post of mine, about one way of doing the imagining-on-paper which is planning a novel, useful:
http://emmadarwin.typepad.com/thisitchofwriting/2010/05/help-yourself.html <Added>Meant to say, thanks for raising the question of planning, Ada, as I've been meaning to tweak that post on planning, and hadn't got round to it, and now I have.