Have to admit I've been reading some romantic suspense in the last few weeks, studying the ways various authors insert that necessary bit of creepiness in their stories. The best ones slip it in so gently you don't suspect it's coming so it catches you flatfooted and unprepared.
Some telegraph it, try to explain the significance of the creepiness, and therefore take some of the punch away. It can work. But it doesn't work quite the same way.
Others bludgeon the reader with enough pukeable gore to turn the reader off. Now I know people in all walks of life have to deal with all sorts of things, but when you're pushing the gore quotient, then I think the romance has to be really, really fine. I'm not talking about sex, either. I'm talking about a hero/heroine you can really get behind and root for because the reader needs something to counterbalance the gore.
Creepiness, the best creepiness should make the hair on the neck stand up. Of course, the reader's own phobias can add or subtract some of the creepiness balance. For instance, some people were completely creeped out by the carnivorous spiders in my book, Kama Sutra Lovers. Other readers commented about the overall creepiness of the sharcrabs in the same book. And one woman wrote to tell me she thought the scene that faded away with the bad guy screaming was pretty darn creepy.
I suspect that horror hits closer to home if it's perceived as possible. If the book is a fantasy, it's less likely to have a sense of immediacy than if it's a contemporary tale in a setting that readers can relate to. That is possibly why a tale such as Fatal Attraction is scarier than say...Friday the Thirteenth.
In any case, be warned. I've been studying hard these last few weeks. No telling what I'll come up with when all the ideas roaming around in my brain get together. Sleep well, tonight!