As a thriller writer, I'd never intended to do a "series" character - a hero or heroine who returns in book after book. To me, this was too much like TV. Each book is just a new episode in the character's life, rather than being a uniquely important event. Besides, I thought it would be boring to do the same character over and over. What a drag.
"Series" books are big with publishers, though, because they're considered easier to market than standalones. Over the years my publisher would suggest doing a series now and then, but I always resisted.
Then a few years ago my then-editor, Doug Grad, suggested that since I'd created several female protagonists who lived in L.A., I might try having two or more of them join forces. This idea sparked something in me, and I ended up bringing together the heroines of my books Next Victim and The Shadow Hunter. They teamed up in Dangerous Games, and now they're back in my just-released book Mortal Faults. (Buy it now at Amazon.com! Okay, sales pitch over.)
Someh0w, using two characters instead of one made the idea of a series more palatable to me. I liked the dynamic of the two women, who are opposites in most respects, and therefore continually at odds with each other. The ongoing conflict makes it easy to write their dialogue and to develop new situations that test their tenuous friendship. In Mortal Faults the test is particularly severe.
The characters can be defined quite easily by contrasting one with the other. Abby is self-employed, a freelance security operative who makes up her own rules and violates any laws she finds inconvenient; Tess is in a management position in the FBI, and she believes in following procedure whenever possible. Abby is hyperkinetic, sexually adventurous, and a night owl; Tess is calmer, much more cautious about entering into relationships, and (for the most part) an eight-to-six office worker.
These differences extend to other areas that aren't always spelled out in the books. In just about any area of life, I can tell you where these two ladies stand. For instance, Abby is a Democrat; Tess is a Republican. Abby is secular-minded with a touch of New Age consciousness; Tess is a traditional (though briefly lapsed) Catholic. Abby likes exotic foods but minimizes red meat in her diet; Tess is a meat-and-potatoes gal. Abby avoids caffeine because she's already so jittery; Tess drinks pots of coffee to get through her paperwork-filled day. Abby is the type of neighbor who plays her music too loud at all hours of the night; Tess is quiet, reliable, and will water your plants when you're away.
Generally, I can write Abby just by projecting a set of preferences that are the opposite of how I actually feel. I don't like noisy, crowded places - so Abby does. I don't drive a flashy car - so Abby does. I don't obsess over movie trivia - so Abby does. She is the anti-me. Whatever I am, she's not, and vice versa. At least, this is largely true. But there always has to be some common ground between a writer and his fictional characters. In Abby's case, it's probably her tendency to keep to herself a lot. I do that, too. (So do most writers, I think.)
Because she is a more extreme personality type, Abby is easier to write than Tess, who is more "normal" in most respects. I can relate to Tess. She sees the world basically the way I do. That's why, when people occasionally ask which of the two women I like better, I have to go with Tess, even though Abby seems to be the favorite of most readers. Tess is not as lively as Abby, true; but she's ultimately a better person - more mature, more responsible, more capable of seeing the larger dimensions of her work. Abby reminds me of a teenager who hasn't quite grown up - hyperactive, reckless, narcissistic. She is charming but, like any narcissist, potentially quite dangerous. A world of Abbys would be a world in chaos, but a world of Tesses would work okay.
Really, though, we need both. Or at least it seems my books do!