Ah, yes. Characterization and character development is a tricky business. Think about your favorite character for a moment, Mask- what do you like about the character? What do you dislike about the character? Characters are like people- they too have strengths and weaknesses.
The difference between a good character and a bad character stems largely from those strengths and weaknesses. It's almost like creating a character on an MMO- if you do not stat your character correctly, they become unusable in the context of the game. In writing, it is no different; a believable character has virtues and vices, strengths and flaws.
Before you even get into execution, that is basic character groundwork that must be laid out. I'll give you an example-
Peter Parker/Spiderman. He started out life as your typical nerdy bookworm at first- shy, socially inept, not in any way shape athletic, and is apathetic in the beginning. However, he was also quite intelligent and clever, not to mention all of those nifty powers he gets from that spider bite.
That's all for now. I hope that helps you! :>