In a dark and stormy night... sitting in front of the computer, an idea struck you like lighting. It's not just any old idea, it's the idea for the greatest novel in the world. It's so great that you've got to write it down now! .....
When you've got an idea for a novel, you don't just write it down immediately. You've got to wait and let it grow. Like a seed, you've got to nurture it and take care of it, until given enough of time, it will sprout into a full novel idea.
I believe that once you're struck with an idea for a novel, you have to wait at least six months, before you start working on the novel. Because within that time, your idea will grow and stretch and do the hokey-pokey in your mind. When that happens, the idea will become something even more amazing.
I should know, because the novel I'm writing was pretty much a generic idea when it started out. Now, 5 years later (yes, I've been letting the novel idea grow in my mind for 5 years. And yes, I was procrastinating), my novel idea has grown into something more awesome.
So when you're struck with an idea for a novel, you shouldn't just jump into it right away. You should let it sit in your mind for a minimum of six months. During those six months, you should write down the random ideas that come to you, which can add more depth to your novel. After that, outline those random ideas and then turn it into a novel (I know not every writer outlines his novel before he writes it, but I'm the kind of writer that likes to outline his ideas).
I hope this post was helpful to all you writers out there,
P.S. -- Letting the novel idea grow in your head shouldn't be an excuse for you to just sit there and procrastinate. Even though I'm guilty of this, that doesn't mean that you should be too. So at the end of those six months, start writing!