Georgie Porgie pudding and pie,
Kissed the girls and made them cry
When the boys came out to play,
Georgie Porgie ran away.
~ traditional nursery rhyme
When I first started writing fiction, I wrote pretty much exclusively fantasy and swords and sorcery. My characters fought and worked magic and found themselves trying to survive in all kinds of weird situations. But the one thing they did not do was kiss. Not even a peck on the cheek. In fact, my characters hardly ever even touched each other, unless it was a punch to the jaw or a sword through the midsection.
It took a long time before I felt comfortable allowing physical contact between my characters. After years of honing my craft, I finally allowed my characters to kiss. These days they not only kiss they . . . um . . . do a lot more than kiss. ;-)
The amount of heat in your novel is up to you, as the writer, and what your story calls for, but it all starts with that first kiss.
You can’t just have your characters going around willy nilly kissing other characters, there needs to be a reason for the kiss. Imagine if someone you’d never seen before, and to whom you were not attracted, came up and laid a big, fat kiss on you. Would it turn you on, or would it disgust you? My guess is the latter. There has to be some kind of pull between your characters. This can be a slow build up or love at first sight, but they must be attracted to each other on some level before their kiss will be believable.
There are many kinds of kisses; you need to choose the right one for both your characters and your scene. A kiss may be shy, or hesitant. It could be stolen, or confident, or demanding. Or it could very well be the height of passion. Keep this in mind and write the actions that go along with this kiss.
A kiss in a dark alleyway is completely different to a kiss on the top of a Ferris wheel or a kiss in front of a fireplace. Write the scene with as much detail as it takes for the reader to easily picture it. The setting and scene of the kiss can be as important as the kiss itself.
The age and experience of the two kissing need to be taken into consideration as well. The first kiss between two inexperienced teenagers will be much different than the first kiss between two experienced adults who hooked up at a bar. Or perhaps one character is experienced and the other not.
Engage all the senses when writing the scene. A reader is more involved when his or her senses are stimulated. Let them feel the heat of bodies pressed together, hear the quickening of the breath, smell the cologne he used or the perfume she did. If tongues are involved then taste will be involved as well - a hint of coffee or wine, the stronger taste of beer or garlic bread.
And don't forget to let the reader know what your characters are feeling. Maybe your characters were fighting and it's a hard, angry kiss. The kiss could take one character by surprise, or maybe both. Maybe it was much anticipated. It can turn the legs to jelly and be completely overwhelming. Or maybe, despite the anticipation, it could be a complete disappointment.
Remember that writing a kissing scene isn’t much different than writing any other scene. It’s full of dialogue and description, people moving around, making decisions, and carrying out actions. But above all, the kissing scene should move the story forward.