I've been noticing as I'm writing crime book three (deftly slipping in excuse about not blogging for so long) how a story grows in unexpected ways. I don't plan much, but I do have a point to point scheme to follow, usually covering the next two or three chapters.
What's interesting is how things change as characters come alive on the page. For example, I had an image of one major character as a secretive quiet sort, and she suddenly exploded into a creature of passion and temperament, surprising me as well as the other characters on stage at the time. The story immediately took off.
This is the key, I think. You've got to allow your inner writer to do it for you. You put the characters on the stage, start the ball rolling, and let 'em rip. For me, that's when writing is a joy.
Of course it doesn't last. The very next thing I did after that was ignore my point to point plan and promptly write myself into a corner. Heroine was in the wrong place at the wrong time, there was vital info missing and I couldn't get out of it without backtracking on the scene I was then writing and putting in the missing one.
What's happening here is the interweaving of the plotter with the writer. Both jobs are needed, but the wise plotter knows when to let the writer have his head. Plotter steers, writer grows the story.
Even if you don't plan at all and fly by the seat of your pants, there is still this game of keeping the two tasks working in tandem.