How was the overall storyline of Lost constructed?

Question

The storyline of Lost stretched over six seasons and had many twists and turns. Many ponderous issues were swept under the rug and forgotten (e.g. why don't we hear much about the numbers or about baby/mama problems in the later seasons?). But a lot more of these interesting details were skillfully woven back and forth through the show, sometimes having episodes explain things from two or three seasons earlier.

How much do we know about how the show was planned out? Did the writers have a plan for the smoke monster being an ancient disembodied man, that when Jack saw Christian in the early episodes it was really MIB, and about his brother Jacob, from the beginning? Did they have the basic existence of and relationship between the Others and the Dharma people figured out? Did they have the role of Desmond marked out from the beginning?

Or did they just randomly come up with crazy amazing ideas for a few seasons and go "Shoot, how do we tie this all together now?" What kind of behind-the-scenes info is available?

Answer 1

As with many scripts, the start with a beginning and an end and then make up the details as they go along.

From an interview with the showrunners:

Carroll: How much of what we’re learning now in season six was figured out in the first season, versus, say, a month ago?

Lindelof: There’s no empirical answer to that question. The way we got through the first season was, if we introduced a mystery, like a polar bear running through the jungle, or a hatch that was discovered in the ground, we had to know what the resolution of that specific mystery was. And in the episode-to-episode writing of the show there’s an enormous amount of discovery. I think one of the most profound lessons I’ve learned over time as a show runner is that the more you listen to the show, the better your show. We watched our characters interact, and that would influence how we put them together.

and

Carroll: It’s one thing to have a hypothesis and even figure out what the predictions of that theory are, but then it’s going to be shaped by what you observe in the universe.

Lindelof: We have to have the answers to the mysteries so that there is something to work towards, but what we don’t have are the stories. J.K. Rowling can sit down and say, here’s how Harry Potter’s parents were killed, and here’s who killed them, but how am I going to reveal that information to the audience in the most emotionally impactful way? So we know what we want to do, but we have very little idea of how and when we’re going to do it.

Not mentioned in this particular interview is the fact that JJ Abrams originally worked with Lindelof to create the show's "Bible" which is typically an overview of the world, characters, general plotlines and any major crucial details.