A common scenario is where the two individuals function in proximity, such as co-workers or in a group of friends, but do not have sex to avoid awkwardness or for other reasons.
[1] Sexual tension often occurs between individuals when the relationship is close and often flirtatious, yet the people involved deny or repress their feelings for each other.
This longing is often suggested by incidents of intimacy; for example, when two characters are alone and in proximity (or actually touching), yet desire is never explicitly expressed.
Another common theme is for characters to develop an interest in each other over the course of the story, and if this is expertly done, the audience can become aware of the growing attraction.
However, as several television series have discovered to their sorrow, tension dissipates when characters come together in sexual harmony.