Here’s a simple ingredient for compelling characters: Eccentricity.
Sol Stein, in Stein On Writing, lays the smack down:
Eccentricity is at the heart of strong characterization. The most effective characters have profound roots in human behavior. Their richest feelings may be similar to those held by many others. However, as characters their eccentricities dominate the reader’s first vision of them.
If you were to examine the surviving novels of the twentieth century, you would find that a majority of the most memorable characters in fiction are to some degree eccentric.
This is not a tough argument to make. Imagine your favorite characters in novels you love the most. Boom, eccentric.
The problem is that our main characters are a shadow, a fragment, of ourselves… the writer. And inexperienced writers are afraid of what they might reveal, afraid of seeming ridiculous or perverted.
So inexperienced writers create characters who just want to get through this (whatever “this” is), who just want to live a normal life.
Bad news for those writers: The weight of literary history is against them. Examples:
Melville’s Captain Ahab, Fitzgerald’s Gatsby, Salinger’s Holden Caulfield, and Joseph Conrad’s Kurtz all possess eccentric personalities and drives.
Quirks are everywhere, from Indiana Jones’s hat and whip, to Harry Potter’s scar and glasses, to Manny’s mechanical arm (that’s Heinlein’s The Moon is a Harsh Mistress).
Shall we include unusual background, or legacy? That’s gets us Harry Potter again, and Luke Skywalker.
I’m not saying that eccentric characters are all you need. If you do have nicely freakish characters, your job isn’t finished. Now you can try for some character complexity and tap the power of your readers’ envy.