When the words flow without effort. Typically, if I am struggling to write, there’s a good reason. Most likely, the story either lacks potential or I’m not in the right time or place to write it.
When the words flow, I know I’m on the right track with a good idea. In these situations, the characters take over and I serve only as a conduit to put their thoughts and actions into print. This phenomenon happened when I was writing The Drift. As if they were living, breathing entities, my characters led me through their stories, step by step. If I were to stop writing at a point that left them at a cliffhanger moment, they would haunt my dreams for resolution. When that happens, you know you are onto something good.
Another tell-tale indicator of a solid idea is when the plot unfolds before your eyes. If you can envision the ending before you even start, then you’re probably onto something good. If you have to work hard to develop it, then you’re probably off-kilter.
One thing that might help when you are developing an idea is to storyboard or plot it out on paper. You need more than a beginning and ending; you need development to the climax. Plotting it out can help with this by giving you a visual representation of what you are trying to accomplish.
Regardless of genre, the goal is to guide the reader to the ending, not reveal it upfront. This is especially true in writing thrillers, but it applies to all types of writing from romances to comedies to adventures. The delight is in the journey, not the destination.
That said, the destination does matter. You always want an ending that leaves the reader satisfied and wanting more. An unexpected climax is a great way to get there, but it can be something as simple as the good guy wins in the end. On the other hand, shock value is powerful. If you have your readers expecting one outcome and you provide another, that can have a major impact.
Happy endings, while fine in moderation, are overrated. Sometimes, it is more realistic and satisfying to have a traumatic ending. You have to be careful with this tactic, though. If your entire book has gotten your readers invested in your lead character and you kill her off at the end, they might skip reading your next book. I recommend you play this one with care and tread lightly.
The bottom line is that if the words are flowing and the plot is unfolding, then you are probably onto something that readers will love.