Do I Have To Answer Every Single Question Before I Start Writing This Screenplay?

There's basically two school of thought on this: the "yes you do" theory, and the "no, no you don't." I just don't know which one is the right answer. I can see the merit in both. By answering all the questions ahead of time, nothing is left to chance. But the problem with answering it all beforehand is that nothing is left to chance. Have I made the dilemma painfully obvious yet?

I mean, the core questions are answered. Who are the main characters, what is their goal, what's the ending? The more minute details are really what's left, like for example, what exactly is plan A, and why does it fail? Okay, that seems kind of big now that I wrote it down. I mean, I have a general idea of what plan A entails, but no real details. Same with plan B, actually.

Okay, so I probably have to figure those two plot points out a little more before starting, but where does it end? At what point do I stop answering questions and start writing this thing. What needs to be planned, and what needs to come organically out of the story writing process, if anything? I like the idea of knowing everything about the story before I write it, because then I think it would go a lot smoother, but at the same time I would like the story to kind of take me where it wants to go (oh that sounds sooooo pretentious).

Answers people, please.