How do you know if an idea will work as a novel, as opposed to a short story? For me, a story has legs as a novel if I can imagine what the second act will look like, at least in rough terms. The answer also involves cricket bats.
In David Mamet's Three Uses of the Knife, he recounts a joke from the Algonquin Round Table:
A couple of guys are sitting around talking. One says, “How’s the play going?” The other says, “I’m having second act problems.” Everybody laughs. “Of course you’re having second act problems!”
In Robert Altman's The Player, conniving movie executive Griffin Mill (played by Tim Robbins) makes a show of allowing a rival executive (played by Peter Gallagher) to steal and take credit for a movie idea. Mill confides later to a junior executive that the idea is going to blow up in his rival's face because it has no second act.
Like carbon monoxide, the second act is the invisible killer. In my early, desultory attempts at writing a novel, I would often start out with what I thought was a great idea with a strong opening and then I would hit the wall. That wall was the second act.
The second act is where the hardest work in writing a novel gets done. Nothing about writing a book is easy but, relatively speaking, creating a strong conflict in Act One is not so tough. Early on in thinking about a book, I usually have an idea of how to bring my conflict to a conclusion in Act Three that is, hopefully, decisive and satisfying. But Act Two is about grappling with that conflict and making it interesting and eventful. Act Two is more like life, which is not about dramatic precipitating events and grand finales -- it's messy, and it's about struggle.
In The Real Thing, the uber-clever Tom Stoppard came up with a wonderful metaphor for writing that has stuck with me ever since I first saw the play:
"This thing here, which looks like a wooden club, is actually several pieces of particular wood cunningly put together in a certain way so that the whole thing is sprung, like a dance floor. It's for hitting cricket balls with. If you get it right, the cricket ball will travel two hundred yards in four seconds, and all you've done is give it a knock like knocking the top off a bottle of stout, and it makes a noise like a trout taking a fly ... [Henry clucks his tongue to make the noise.]
What we're trying to do is write cricket bats, so that when we throw up an idea and give it a little knock, it might ... travel ... ([He] picks up the script.) Now, what we've got here is a lump of wood of roughly the same shape trying to be a cricket bat, and if you hit a ball with it, the ball would travel about ten feet and you will drop the bat and dance about shouting 'Ouch!' with your hands stuck into your armpits. (Indicating the cricket bat.) This isn't better because someone says it's better, or because there's a conspiracy by the MCC to keep cudgels out of Lords. It's better because it's better."
For us Americans, this may make more sense if you replace the cricket bat with a Louisville Slugger, but Stoppard has put his finger on why writers spend so much time crafting the early chapters of a novel. It's like fitting together the wooden pieces of Stoppard's cricket bat. The point is not whether the bat produces a loud crack when the ball is struck (a great first chapter), but whether it will send the ball (the protagonist) arcing through the second act to land with a satisfying whump in Act Three.