Some time back, I had the pleasure of attending a workshop given by best-selling author Brenda Novak. The topic was Romantic Suspense, and since that's what I like to read and write, I figured I could pick up some good tips from one of the masters of the genre. Here's what I learned: A romantic suspense must be ONE story, not a romance and a suspense. They have to be dependent on each other, and the outside tensions have to be equal for both the suspense and the romance. One cannot exist without the other. Things that affect the suspense plot will also impact the relationship.
In a suspense, the timeline is usually short. Building the relationship over a short time has to feel plausible.
There's usually a lot of research involved, and you have to do enough to write with confidence about your subject. However (and this point was reiterated in the FBI workshop I took later), she said it's better to go with a widely held belief rather than confuse readers even though you're "right." This is problematic, because so many readers watch the CSI type shows and believe what happens there is the truth. They'll assume you're wrong, even if you're right.