Part of character development has to include eccentricities and things on which my characters enjoy spending both time and money.
I have trouble with heights. Strike that. I have a lot of trouble with heights. That's one reason why our upper deck is pretty much the domain of my husband. I'm okay as long as I'm looking out at the mountains, but looking down? Three stories? It's like a magnetic spiral with deadly appeal.
If my character has this same phobia, giving her a hobby of rock climbing would not be a very smart move on my part. But I definitely want to consider the possibility of having her out on a girder of a high rise under construction with a menacing something, or someone, right behind her. Wowzer. Terrific conflict.
Same thing if my character is afraid of closed-in places. I will want to have him stuck in a tiny cave at some point.
Quirks, used well, can be tools of endearment. The trick is to not stretch them to the point they seem contrived. A quirk for the sake of a quirk doesn't come off well.
Let's take lipstick for example. (Yes, the current news is bombarding my brain.) It's not whack-o to have my character have a need to reapply lipstick whenever she feels anxious. It would be whack-o for her to break out her entire makeup bag and do the same thing. Not only whack-o, but action stopping.
One of my favorite television programs is Monk. Here is a character who is filled with phobias and quirks and yet remains lovable. But Monk falls more under the label of "cozy" as opposed to suspense.
As a suspense novelist, I need to be selective. It may even be the I have the information (with my background character studies) and these personality traits never meet the light of day. Nothing wrong with that.
Phobias, hobbies and quirks can help to round out a character. And provide fodder for conflict, the spice of life. Well, the spice of fiction.
Still reading Kill Me. It's a good book, I've just been busy with anniversaries and entertaining and getting ready for special company.
It's all better with friends.