When writing a sentence (for a book/story) do the endearments Honey, Sweetheart, etc. get capital letters? e.g. "Are you ready, Honey?" or "Are you ready, honey?"
"Click" and "Clack" are capitalized because they’re nicknames—they take the place of a real name. … A term of endearment isn’t interchangeable with a name the same way a nickname is, and terms of endearment aren’t capitalized.
Chicago’s preferred style has always been to lowercase pet names, but you can’t go wrong unless you’re inconsistent, since the issue is guided by preference rather than rule.
A term of endearment does not need a capital letter. However, any term that is used consistently enough to be a name should be capitalized.
A good example of how to do this can be found Fritz Leiber's short story, Kreativity for Kats, which you can read in its entirety online.
Using the name (i.e. capitalized) form of an endearment can be used to emphasize a perspective, i.e. it's a tool you can use, not merely a grammatical rule.
If you are using a term of endearment as a nick-name then I think it should be capitalised (I'm Australian, hence the 's' instead of 'z'): “I love you too, Sweetheart.”
However, if it is not used as a nick-name, then it should not be capitalised: "Jack, permit me to introduce my sweetheart, Heather."