One of the more difficult — and interesting — aspects of writing a thriller, for me, has been crafting the villain — understanding his/her psychology, what makes him/her tick. My villain is methodical, but relatable and flawed, and I wonder if that detracts from the creepiness of the character.
What makes a bad guy/gal scary? Is it the more we know about him/her, or the less? Is it smarts or plain stupidity or ignorance? Are the best villains the ones who started off good and then became corrupted, or those who have always been bad? What do you think? Who has scared you?