I believe it was Jim Frey (www.jamesnfrey.com ) who came up with the principle of maximum capacity as it applies to character building. Characters in dramatic fiction are revealed through their actions when faced with a challenge—and as a writer make certain that your characters meet challenge after challenge. If when faced with a challenge your character does not employ every resource within his power, you the writer face the risk of your reader abandoning you.
I’m a big fan of Elmore Leonard, but his novel, Cat Chaser, is a classic example of two attractive characters acting like idiots. Leonard creates a likeable motel owner, George Moran, and his soul mate, Mary de Boya. Only Mary is the wife of a super dangerous bad dude. My problem with the novel is that I believed that the bad guy was a serious, serious threat, so I didn’t understand why George and Mary took so many chances. Either the bad guy wasn’t a threat or the protagonists were idiots. I found myself holding my breath more than once, but then I gave up. It was like if you two don’t care about your personal safety, why should I?
Back to Jo Nesbo’s Headhunters (did I mention they made the novel into a movie?) : Roger Brown was not a likeable character, but when he faced real danger, he thought of ways to elude his threat that were both surprising and ingenious. Over the span of the novel I grew ever more sympathetic to his cause.
As you have your characters acting at maximum capacity, remember that they must behave within the bounds of their personality. It’s unlikely that the shy accountant will vanquish her stalker in a gun battle (unless you set her up as a sharp shooter at the beginning of your story).
So give your character an obstacle and then figure out how she will overcome it. Writing a list of solutions is a great tool for coming up with surprise. Make a list of 30 possibilities. The first ten might all be cliché, and then by the grace of the Muse, you might hit gold.