im under the impression that a good 75% of the show has been preconceived; at least the general, broad strokes. time travel, smoke monster, others, devil guy, etc. little details have certainly come about (niki and paulo) and there's things we know that have been altered a good deal, like ben lasting more than a few episodes and being the actual leader, jack's death in the first episode, and i think charlie's death was supposed to happen earlier.
