As the Enola lawyers wrote, the generic character traits (their age, emotional vibe, etc) are not protected. Neither is anything "trivial" or "minuscule" - for example, if a cartoonist makes a minor alteration to the way a character's pupils or eyes are drawn, that's a minor detail, not a copyrightable element.
The biggest impediment to using public domain characters isn't copyright, it's *trademark*.
28/