Blogger friend Elena Solodow posted a link to this cool blogging experiment at Elena Johnson's blog. At the time of writing, there were 193 bloggers signed up to post about this topic: How to write compelling characters.
OK, I confess some small feelings of fraudulence here, because I know that characters are one of my weak spots. Along with plot, tension, story arc, dialogue, punctuation...
So who am I to be giving advice here? But I'm a sucker for an interesting experiment, so, in the name of science, here goes...
My piece of advice is simple: Observe real people.
Your characters are people, and people are infinitely interesting. How better to make your characters compelling than to look at what you find compelling in people? Besides, truth is stranger than fiction, so I bet you can find more depth and variety in the real world than you'd ever be able to dream up for yourself. And if you ground yourself in reality, I bet you can envisage a characterstic more fully and therefore write about it more compellingly than if you'd just invented it.
Even your non-people characters probably need to have some kind of recognisable traits for readers to identify with. And if you are trying to emphasise the alien-ness of a character, you need a solid reference point of how humans behave from which to depart.
Of course, I don't mean just copy any old features you see, but rather draw your inspiration from real life.
For example, if you need a villain, rather than drag out a stereotype from the dusty "villains" box on the shelf above your desk, look at some real villains. What motivated Jack the Ripper? Can you capture the ruthlessness of Al Capone? What compelled millions of decent, upstanding citizens follow Adolf Hitler so enthusiastically into war?
Or add depth to a character by using details you've observed elsewhere. I don't mean to base your characters on people around you, but take note of specifics that really caught your eye. Maybe a characteristic gesture or manner of speech, or something about the way they walk. Someone's insistence on dunking their teabag for no more than four seconds when making a cup of tea. Someone else's collection of pysanky lining the bookshelves in their living room.
If you introduce things that truly fascinated you, then you can bring that fascination out on the page. Polish it up to a shine and share it with your readers. But be sparing. You don't want the detail to overwhelm the character. Tease your readers with tantalising glimpses of these gems of insight. I'm sure they will thank you for it.