Ah, sexual tension. What is a decent romantic subplot without it? Initial physical attraction...denial...resistance...and sometimes (but more often than not, a lack of) sweet surrender! All wonderful things to flesh out a romantic relationship in a story.
I just love watching sexual tension develop between characters--I don't care if it is in a movie, TV series, or novel...hell, even in real life! I just like seeing it! There is definitely an "art" to creating this type of tension. An unskilled writer can make it look cliched and contrived, however when sexual tension is done right--wow! I don't think there is anything more explosively entertaining to draw an audience into a story! When folks are screaming: "GET IT OVER WITH AND BANG THE BITCH ALREADY!" you know you're doing the right thing! Haha!
They key to creating sexual tension is to first understand what sexual tension is. Are there different types? What sort of scenarios create this tension? I'll write more posts about this topic in the future, but for now let's cover the basics.
As human beings, we naturally try to alleviate the things that frustrate us. However when we're dealing with sexual tension, the situation or circumstance often prevents us relieving the discomforting feelings. This frustration is what essentially drives the sexual tension forward. This is why sexual tension is inherently more "sexual" than traditional overt sexual displays. Denial and restraint require more mental and physical energy than surrendering to an indulgence. The longer we stay trapped in this unpleasant and frustrated state, the stronger the conflict and tension.
Just like in real life, sexual tension occurs for a variety of reasons. Perhaps two married co-workers develop an interest in each other at the office, or a high school student harbors a crush on their much older teacher. So long as the EMOTION behind the reason is palpable enough to draw in an audience, you can create sexual frustration between characters. Conflict is always entertaining to watch--and readers loved being teased.
One way to create sexual tension is to make one character push forward (or initiate a sexual response) while the other character steps back. Using the scenario from above, let's pretend the seventeen-year-old schoolboy accidentally bumps into his teacher in the hallway when they are alone. The unexpected physical contact makes him blush, and instead of immediately moving away, he lets his hand linger on her shoulder. In this scenario, the student is the one who is pushing forward with the initial contact and so the teacher must inevitably pull back. Let's say she does not want to act inappropriately with this young boy or maybe she does not want to lose her job. So she smiles politely at him, steps away from his touch, and quickly excuses herself to the teacher's lounge.
You can also create sexual tension by making both characters step back or refrain from initiating the sexual response, but this will only work if an external force propels them forward again. Maybe our schoolboy is painfully shy around the women he is attracted to. He knows he could never get away with doing something so bold as the first example, so he keeps to himself and just makes goo-goo eyes at his teacher during class. The teacher is still concerned with upholding professional boundaries and so she does not say or do anything to encourage his behavior. However, now let's imagine the teacher is experiencing financial difficulties due to budget cuts and mass lay-offs at work. Unable to afford her mortgage, she moves across the city into a small apartment--the same apartment our little schoolboy lives. Now that these two are neighbors they are forced to see each other every day in a more "intimate" setting and the sexual tension can move forward again. The external force pushing them together is proximity. They can start carpooling together or maybe the teacher offers to help him with his homework after class. The possibilities are endless.
The thing to remember is that the distance between your two characters must always remain far enough to create frustration while maintaining room for interpersonal interaction. The longer you let this "push-pull" effect control your characters, the better and richer the sexual tension. However, if you let it go for too long, people will lose interest. Finding the right balance is key.