FROM DANZAHOOP
Rhett says, "Frankly, my dear...."
I've read published "Okay, Kiddo..."
So if Rhett had said, "Frankly, Dear...." would dear be capitalized?
RESPONSE
"Dear" is not a legitimate title, but a term of endearment, like "sweetheart" or "poopsie". It's never capitalized unless it's at the beginning of a sentence, even if a character addresses another character directly as "dear". Of course, if someone had the nickname "Dear", that's another matter.
The reason you never capitalize terms of endearment is because ANYBODY can be addressed by those terms. You can call a complete stranger "dear" or "kiddo". I had a friend who addressed everyone by "darling". Consider it from the opposite spectrum - terms of disparagement:
Hey, putz, wake up! - Do you capitalize it if it's not a nickname? Nope.
I'm not going to be late because chowderhead can't make up her mind. - Capitalize "chowderhead"? Nope.
Heck, even if terms of disparagement become terms of endearment, you don't capitalize:
Don't worry, I'll fix it, darling dunderhead.