Karen Rose is another author I admire and respect—and, like Brenda Novak, Karen is on my 'don't mind coming in second to her' list -- this one was in the Lories.
She addressed some of the areas where romantic suspense has different emphases from other sub-genres. Like any other book, establishing the goals, motivations, and conflict for the hero and heroine is vital. However, she attests that in romantic suspense, the villain drives the story.
The villain: Establish the villain's motivation. Some effective ones can be revenge, power, or greed. Give him a personality. Is he antisocial? Sadistic? Dependent? Somewhere, he should have a reason for doing what he does, but she also said some people are just "bad" in general. Is there some pathology behind his behavior.
(Note: If you followed my series on Criminal Thinking, you might also pick up some character help there. Your villain will probably think differently from your hero and heroine.)
Your villain must be smart. At least as smart as your hero and heroine. He must be a formidable opponent, although he can appear to be the man next door. He must also be 3-dimensional. Find his vulnerability.
Things to avoid: The "they" of faceless groups. Find one individual and connect with him. Stupid, bumbling villains. Redeemable villains. Cardboard villains. Villains who appear from nowhere. The motivation of the villain can be the mystery.
Hero and Heroine: their relationship must be threatened by the villain, fed by the suspense. They must stay close to the action. Be sure their occupation(s) give them a right to be where they need to be.
Things to avoid: a heroine too dependent on the hero. Stupidity. A too 'macho' hero. Too much arguing.
Victims: make them human.
Secondary characters: they give dimension to the hero and heroine. Options include: best friend, family, neighbors, coworkers. Secondary characters are great for explaining technology and avoiding info dumps or too much telling. They can provide conflict.
Plot: get the details right, and know where to drip the clues. The romance and the plot have to intertwine. Each scene has to feed both the romance and the suspense. A secondary plot gives a 'bigger book' feel.
Pacing: each scene has to advance the plot. Suspense scenes are usually shorter. A romantic suspense has an urgent mood.
Continuing dilemma for me: some of these gems don't lend themselves as well to a mystery-centered story. More things to work on.