You've either got to find a way to make your continuing characters insteresting without making them maudlin or overwrought, or you've got to put more emphasis on the suspects.
“I seemed to belong to three countries: I had an apartment in Paris, a house in Hollywood, and when I married British theater director Peter Hall, I moved to London.”
“The offhand decision of some commonplace mind high in office at a critical moment influences the course of events for a hundred years.”