One of the things I've always liked, as a reader of Harlequin Intrigues, is the helpful Cast of Characters page you find at the front of an Intrigue. It introduces the main characters as well as the various suspects in the mystery that makes up the plot of an Intrigue.
However, until I sold FORBIDDEN TERRITORY, I always assumed that the editors wrote the cast of characters. So I had a mild panic attack when my editor asked me to work up a Cast of Characters for my book. However, it turned out to be easier than I expected, and I've actually figured out that I can use the cast of characters as a plotting tool.
Usually when I start plotting a story, I have a basic idea of the overall plot (is it an adventure, a mystery, is it gritty or cosy, is it a caper or a thriller, etc.) and I know the hero, heroine and probably the villain. What writing the Cast of Characters forces me to do is think through my characters and their goals, motivations and conflicts. What drives the hero and heroine into the heart of my mystery plot? Who's going to be an ally and who's going to be an obstacle (if not an outright villain)? Who are my red herrings? (Every Intrigue should have at least two or three, minimum, although their strength as red herrings may vary). Who is my ultimate villain?
By creating the Cast of Characters, I not only get a better look at my main characters and the conflicts that will drive them through the story, I also get an idea of some of the steps they'll have to take along the way (in order to meet some of the secondary characters I've created in my cast of characters). It helps me get a mental roadmap of what my protagonists will face over the course of the story. Once that's in place, it's exponentially easier to fill in the blanks of the plot.
So if you're ever floundering around, looking for a road map for your story, try creating a Cast of Characters page for your own story. It might work just as well for a straight romance as it does for a romantic suspense.