I feel like this is an aspect of storytelling that I totally don't have an answer for. I make a lot of characters, and some of them I like a lot, and some of them I develop a lot and yet never feel a 'spark' for, even if they're constructed in a way that should work.
For example, all last summer I was working on a short horror comic about a serial killer. I developed the character meticulously, but I needed him to kill someone. I drew this generic looking blond guy and put him in a leather jacket and decided a few other character traits for this victim, who's only role was to die. One of the traits I gave him was that he was in a punk band...
Flash forward to this year and I have a comic about a punk band, featuring Victim Guy as one of the main characters. He just felt like someone that needed his story told, and it's not neccessarily because he's got an exciting personality- he's grounded, calm and mature. snorefest! But I think he's a far more interesting character than a charming murderer.
I think sometimes the characters that undergo the most development to make them 'interesting' are the characters that are abhorrently uninteresting. I don't care about twisted motives or 'I can't figure what they're going to do next' or 'what witticism' (ok I lied, I like witticism)
I think making characters, and making a ton of them is the best way to kind of figure out what works, and what you as a writer write best. Even then, who knows if other people actually find said characters as interesting as you do...