This reminds me of what Stephen King mentioned in his book "On Writing" (I'm pretty sure that's where I remember this from) - some writers like to structure things out, whereas others - like Stephen King himself - like to just plant a seed and see where it takes you, hope that the story evolves properly and finds its natural conclusion.

Basically, some writers are architects, while others are more like botanists. Both have their strengths and weaknesses, so there's no right or wrong approach.

In Lucas' case - dude just kind of rolled with it in both directions it seems (with more emphasis on botany). Till he accidentally cracked the biggest code in cinematic history...