I urge you to take a few minutes to read Ramsey Shehadeh's short story Creature. After you've read it through once, take a look again at the second section (sections are separated by "* * *"), and look at how the author portrayed Creature's desires, insecurities, and motivations. Consider its wants and needs, its internal and external influences. Look at how much character development the author gives you in just that one paragraph following the section break.
Even if you're not a genre writer, let's think about this for a moment. How much do you know about your villain/monster/antagonist's backstory? How was s/he born or created? How was s/he raised, or not raised? (Tell me about your mother, we hear in a deep Freudian accent.)
Maybe try your hand at the following writing exercises:
- Describe your monster's birth or creation (in the case of my creature, hatching);
- Write a scene in which your monster/antagonist becomes frightening or repulsive even to him/herself;
- Write a scene in which your monster/antagonist fails to overcome his or her evil (or merely naughty) impulses... and does not regret it.
That short story I linked to? Includes all three such scenes. Yowza.
They say that the way to make vivid, well-rounded characters is to remember that every character is the hero of his or her own story. That includes monsters and villians. That includes characters who are self-loathing, or who wish they were more than they are... or less.
I hate to quote Ally McBeal (note to my foreign readers, I swear that show had nothing remotely to do with law as it is practiced in the United States), but there was one episode where a much-abused friend asks Ally, "Why are your problems so much bigger than anyone else's?" And in a rare moment of honesty, Ally replies, "Because they're mine."
No matter what genre you're writing, no matter what species your characters are, no matter how big or small their roles in your stories are... their problems are THEIRS. And that's what going to be important to them. They're probably not out to just be evil or get in your hero's way. They may want power, or love, or just to be left alone.
If those characters of yours get a lot of time on the page, you could do worse than to get inside their heads with them for a while, and see what got them started down those paths.