This blogfest is the brainchild of Elana Johnson, Jen Daiker, and Alex J. Cavanaugh. The list of the other participants can be found here.
Here's my take on what goes into developing compelling characters. Lets start with a definition.
1.tending to compel; overpowering: compelling reasons.
2.having a powerful and irresistible effect; requiring acute admiration, attention, or respect: a man of compelling integrity; a compelling drama.
I tend to focus on the words irresistible effect in that definition. When I sit down to map out (remember, I'm an outliner) what sort of characters will inhabit my fantasy world, I utilize two trains of thought. The first, I give them motivations that the reader can sympathize with. A character your reader can identify with will draw them into your story and heighten their connection. My other approach involves creating extreme characters with outlandish traits or unusual social skills. Taking these type of characters and balancing the WEIRD with the elements that make them relatable can sometimes be a challenge, but it pays off if you work at it.
There is another way to make your character compelling, but I sort of consider it cheating. Its when a plot device sets up an objective that is so compelling, that the character charged with achieving it becomes just as interesting. For example, a young girl is being held for ransom in a deserted farmhouse. If the ransom isn't paid before midnight a bomb will go off in the farmhouse. Our MC is charged with locating the girl before midnight. The MC therefore becomes compelling because the reader is rooting for him to save the girl, not because his/her own internal motivations.
That's just a couple of idea's off the top of my head. Nothing groundbreaking, but still tried and true. Good luck with your own characters!