Running With Enthusiasm Versus Building It

I know that this will qualify as the "duh" statement of the year, but writing a book is a lot of hard work.  If you're not into the project, it'll drag on and on, and you won't turn out a very good product.  That will lead to people writing you off as a hack, never buying your books again, your kids not going to college, and, eventually, to you living under a bridge while hoping someone tosses a few scraps of bread to you.

(Okay, that last bit is harsh and phony, but it felt good to say)

When it comes to enthusiasm, the real question is just how enthusiastic you need to be at the start of a project, or whether you can build enthusiasm over time.  I have two big ideas in my head - the first is the book I'm currently working on; the second is the prequel story to a novel I finished last year.  The latter is something I've been spinning over and over in my head since before I started writing Homecoming.  We writers have fantasies(these fantasies form the foundation of our works), and they sometimes consume us.  The fantasy I've been spinning over ni my head is about the character the plot in that novel revolves around, a man who took humanity and brought them back from the brink.  I've walked through his struggles with losing his family and how he pushed people into fighting back before eventually escaping Earth in order to survive.  It's a novel I'm definitely going to write, and, in fact, can't wait to get to.  This plot has been in my mind for several months, so I've seriously considered pausing in my current project to get this other one done.

With Onyx, on the other hand, I've always been enthusiastic about the general story, but I never had specifics.  It felt draggy for a while.  Then, like a shot out of the blue, I find myself into it.  My enthusiasm for this novel built as I wrote more of the story.  I even wish I could write a little faster so I can find out what happens(a little known secret of writers - we're sometimes just as surprised as the audience as to what happens).  This didn't occur all at once, but rather manifested as I got deeper into the story and got into a groove in writing it.  Since it has horror elements, and I haven't done anything close to that in a while, I've had to relearn what makes a story suspenseful from a horror standpoint.  I rediscovered that in the fourth chapter, and now I'm dying to get back into it and see where it takes me.

So, as a writer, do you always go with the project you're most enthusiastic about, or do you take the enthusiasm you have for an idea and try to build it up as you go?  The momentum created by Onyx has answered that question for the moment, but what happens next time?  It's something I'm going to have to give considerable thought to as I move forward, and I'll bet I'm not the only writer this affects.