I think the number one way to make characters interesting is to give them lots and lots of depth, and I think the best way to do that is how they interact with other characters, and situations. In what situation are they happy, angry, nervous, scared, determined, can't think straight etc. What's important to your characters? What/who would they go out of their way to help.
//shameless plug: I've been told that Toxic For You has really realistic characters, because all of them have flaws, they all act differently depending on who they're with, different people bring out different traits, and that's how people are, its not a two-faced thing, depending on your comfortability with others, changes how you act.
So you can do all those over-used tropes, love triangle, hero-revenge, etc, if you give each of your characters their own personality, responsibilities and sense of moralities. not some cookie cutter "this person is shy therefore they're shy with everyone in their life"