It's not an easy thing to pinpoint. It may take the fifth draft of the book before it truly hits me. Of course I then go back to make sure it's not just in my head, but also on the page. The only way to find it is knowing my character. Writing is circular in that way. Need a plot? Find out who your character is. Need an overall goal for your novel? Find out who your character is.
So how do you find that one thing? Unfortunately it doesn't show up in the ms color-coded or followed by !!!! to emphasize it's importance. Oh, how I wish, because it can be anywhere. And sadly, it's like porn you know when you see it. I've written or read it and something in me paused. I come across it and all the layers I've surrounded my character with makes sense.
Let me see if I can make this clear as mud with an example:
I have this hero. He's all business. He grew up poor, but his push has nothing do with not being poor ever again. At first I thought it had everything to do with his father being murdered. Losing someone in such a violent and sudden way can skew you in a way that can't be defined. For him it made him focus with a tunnel vision that made him torture himself and refuse the lighter side of life. So, of course he wouldn't stop to smell the roses. And, of course pairing him with someone who manages to do everything but focus would drive him insane.
In a way I was right. Losing his father was the One Thing, but only the top layer.
Scared of losing someone else he loves.
That fear turns into him not connecting with others in a deep and intimate way.
The heroine's way of living is his idea of worst nightmare. He pushes her away and by doing that pushing away the very thought of intimacy.
Yet when they get closer he tells himself their attraction, their compatibility is out of his control. When it's really him finally rejecting the idea he doesn't want intimacy.
Then I came across this:
He could have Lynne who made him forget reason. Forget his purpose for being in this town. Make Nate forget the grief that wanted to choke him at night when he was alone.
Out of context this makes absolutely no sense, but when I wrote this I had to stop. Yes, he wants intimacy and is sabotaging himself in order to get it. It's something he craves, but in order for him to have it he has to let go of the grief. The same grief that goes away when he's with the heroine.
His One Thing is letting go. He believes if he lets go of his goal (the goal is the physical manifestation of this ---->), lets go of the grief he will somehow forget his father and making his father's life in vain. It's the one thing he admits to himself when no one else is around--he wants to stop grieving for his father. He secretly wishes that constant ache would lessen.
No the realization isn't ground breaking, but everything about my hero made sense--why he was so cold, why he was fighting to death for his goal, why loving the heroine was the absolutely last thing he wanted to do (not for the idea of loving someone is scary, but what he would have to give up in order to love her). The wonderful thing, it also helped me see what the ending needed to be.
Which is the One Thing I'm still looking for with the new story I talked about. Sigh. Needle in the haystack, anyone?
Have you found your character's One Thing?