Develop your world. Create a mythology for the paranormal elements in the story that is internally consistent and at odds with the normal world.
Create a strong alpha hero and an equally strong female lead. Each should be compellingly attracted to one another by nature, yet have story goals that are diametrically opposed--for example, a criminal and a bounty hunter.
Place both main characters in a plot that brings them into direct external conflict over life-or-death matters. Make their underlying attraction irresistible, providing strong internal conflict. Start the story with intense conflict and attraction between the characters, and raise the tension with each subsequent scene.
Maintain a relentless pace throughout the book, with something exciting happening in each scene, while still ensuring each scene is well-grounded in time and place right away. Make sure each scene is as tight and tense as possible by eliminating unnecessary exposition.
Include mind-blowing sex as early in the book as is possible. In a Nocturne, anything goes. Ensure that the stakes are raised and the plot moved forward in each sex scene you write.
Finish the novel with a happy ending, including a solid romantic commitment between the hero and heroine.