Writing a series can be an asset as well as a hindrance. An asset because your readers look forward to each new novel and your continuing characters. But you can become bored with your character(s) as Agatha Christie did with Hercule Poirot. In fact, she came to hate the arrogant little detective.
My protagonists have become old friends that I enjoy tuning into each day to listen in on their conversations, no matter how scatterbrained they happen to be. But I've also become bored at times writing about my two older women amateur sleuths, who get themselves into situations that I have a difficult time writing them out of.
I don't outline, unless it's a nonfiction book, and my characters have free rein, so they lead me on some wild adventures. Murder at the Mansion is my wildest novel to date, with my protagonists, Dana and Sarah, running for their lives from Wyoming to Texas, Alaska, Colorado, and back again.
The more I get to know my characters, the more I trust that they won't paint me into a corner or refuse to do what I want them to. In my new release, some of the people they helped to place in prison return to seek revenge, and Sarah decides to dissolve a hasty marriage that she regrets. None of these things occurred to me when I sat down to write, so I blame the devious minds of my two protagonists.
Combine mystery, humor, romance, murder, a quirky character or two, and you have the Logan and Cafferty series, which I hope to continue writing for quite some time.