In his book "Story" (see Resources), screenwriter Robert McKee describes the need for "opposing goals." Essentially, this means giving each character in a romantic scene an agenda, something they want to achieve by the end of the scene. Giving characters opposing goals creates conflict, which is essential for any romantic drama. There is nothing interesting about two people falling in love instantly.
Just as each character needs an opposing goal, it is important that protagonists be distinct in other ways as well. Hollywood story consultant Christopher Volger discusses the need for archetypal characters in his seminal work "The Writer's Journey" (see Resources). Vogler argues that screenwriters must develop characters we all recognize and then subvert the audience's expectations for those characters. Most romance novels and films feature characters in opposing predicaments. One is rich, the other poor. One has power, the other does not. One is popular, the other a loner.
The characters in your novel or screenplay must never be "knowing." In a romantic comedy, for example, characters must never think their situation is humorous, otherwise the audience fails to empathize with their predicament. As screenwriter Michael Hague writes, "[The characters] are desperate to achieve their goals, and terrified by the conflicts they face. When the people on the screen are laughing, the people in the audience aren't."
Avoid cliches when writing love scenes. McKee describes the need for "reversals." The writer must always set up the audience's expectation for one thing to happen and then do the opposite. These constant reversals, in both dialogue and action, keep the audience engaged in the unfolding love story. The audience might know the two characters will end up together, but they don't know how the two characters will end up together. That is what keeps them watching or reading.