This is an interesting question, and one I’ve asked other writers in the past. If asked, would you be able to state your hero’s favorite food or color? Your heroine’s favorite memory from childhood? The inciting event that helped mold each of their psyche’s? This is heady stuff and, I feel, very valuable to know.
In a tangential way, this goes back to whether you are a pantser or a plotter. I’ve admitted I plot everything, and that includes having full disclosure from my characters before I start typing. I need to know what makes them tick, what they like, dislike, loathe. What turns them on in life and what turns them on sexually. I need to be able to think in their heads when they are speaking, know what their reactions to events and circumstances would be based on past behaviors and motivations. If they have the capacity to change, and why or why not. I never want a reader to say “he wouldn’t say that!” or “Where did that come from? She’s never said she feels that way.” That’s cheating the reader, and will ultimately disappoint them.
I’m nosey. No ifs, ands, or buts about it. I could talk to a rock and make it answer me. I’m that way with my characters, too. When I’m envisioning my heroine, I know what she looks like, who she got her upturned nose from, if her earlobes are attached or not. I can tell you how she felt when she wore braces and where she kept her diary hidden from her mother when she was 12. I know her secrets, her longings, her desires. With my hero, I can tell you how he felt when he was losing his virginity, who he resembles in his family tree, and how much money he has in his checking account. And I know the answer to this one very specific question for both of them: who would you die for. This sounds a little obsessive and believe me, it is. But the only way I know how to adequately show my characters to the readers is by knowing what their actions and reactions will be, and why.
I heard someone say once that you should write up a series of questions for your characters as if you were going to be speed dating them. You can learn a great deal about people by asking just a few very well pointed questions. Like I said, I’m nosey. The more I know, the better I can draw the character. The better the character is drawn, the more believable (s)he will be. And ultimately, the more believable your characters are, the better and more cohesive the story you can devise for them will be.
So, here’s to nosiness. In writing, it’s a good thing.