As soon as the first was done and dusted, critiqued and mauled, I started the second and third as more-or-less place holders for a proper bit of writing. Just enough to give me an idea what I was really trying to say and what I was actually saying. Sometimes the latter discovery was more valuable that the former pre-conception.
Anyhoo, then I re-wrote the beginnings of the first again and discovered layers I'd been too timid to pursue the first time around. I switched a key dramatic point, 180, which led to a discovery of one of the characters' motivations that I'd been completely unaware of. This is the fun of writing, I think. You can throw yourself curve-balls and find yourself in completely unexpected territory.
Any-anyhoo, I now have two opening chapters and a decision to make as to which comes first. Oh, and a new character I don't know what else to do with quite yet. Fun and frolics ahead, no doubt.
Incidentally, and this is probably ot, a bit, but I put off writing a book at all for decades. I was pretty sure descriptions would be my downfall, those un-entertaining bits that have to happen between lines of dialogue. Once I started, though, my downfall was not in writing them so much as in making them clear. On re-reading one of my first chapters a couple of days ago (written 2005), I found myself asking a couple of times, "Now, what did I mean by that?"