I talked about the method of using one specific detail, rather than many, to show readers what makes that character tick, a task I explain in Growing Great Characters From the Ground Up. In specific, I talked about the detail that defines the main character in the novel I'm currently working on.
When you gave the example of the (defining detail), I thought of a bigger fear of abandonment. That could definitely work in my story. But then that is where I have decided to make the second character the main character. I can actually see the story moving now. Unfortunately, I was busy taking notes and do not recall how the defining detail) comes into the scene. Is it her own discovery as an adult, a flashback of when she was little? I'm trying to figure out where to place the abandonment scene. Maybe I should let the character discover herself as the story goes along.
I wrote the defining detail scene as a prologue and then decided that since I know what the detail says about what makes my character tick, I don't need to show that scene to readers. So the answer is no, you don't have to tell the reader what the defining detail is.
What's important is that you show the character acting/reacting based on that defining detail — her nature — rather than according to what you think she should do.
To make sure you understand, my character doesn't know the spatula is her defining detail. Nor does she know what that says about the basic, inner rule by which she lives. What's important is that I know the rule and so know why she does what she does, information I then pass on so readers can interpret her actions and decisions.
Thanks for the question, Karyn.