Everyone loves a good bad guy/gal, and so the Writer's Digest folks (in this case, guest author Brian Klems) offer some tips on how to flesh out your antagonist:
Luckily, transforming your antagonist from a one-dimensional paper doll into a force to be reckoned with—and remembered—is completely possible if you implement a few simple but powerful methods for creating antagonists and expanding their roles. You can build a worthy adversary during the outlining process or beef one up when you revise your already completed draft. It’s never too late.
He breaks it down for us in 6 steps. I like step 3, actually, as something that seems obvious to me only after reading it. It's tempting in Science Fiction, for example, to face off against Big Evil Corporation. But it's not really that satisfying when you think about it.
- Remember that Antagonists are people, too.
- Eschew the totally evil antagonist (except, possibly, in some horror or monster stories).
- If you’re tempted to say your antagonist is a corporation, disease or war—don’t.
- Make your antagonist at least as smart, strong and capable as the protagonist.
- Keep the tension strong when the antagonist is a friend, ally or loved one.
- If your antagonist remains hidden for much of the story (as in a mystery), give him proxies or let him work behind the scenes.
Read the details of those points here: http://www.writersdigest.com/online-editor/6-ways-to-write-better-bad-guys