This approach to communications theory assumes there is generally one specific purpose to any communications endeavor. Things are expected in such endeavors as job interviews, first dates or small talk at a cocktail party. However, violations of these expectations can work to enhance or demean the status of the violator.
Predictive expectations are those that are built into any specific communications endeavor. In a job interview, for example, certain modes of dress and speech are "built in" to the nature of the institution. It is expected and provides a certain degree of institutionalization and predictability to any situation similar to it.
Prescriptive expectations are precisely those modes of behavior, both verbal and non-verbal, that are assumed to be a part of the institution of communication within a specific context. There is a sort of "script" that those in such endeavors are to follow. But the real question is whether the violation of such prescriptions can work in favor or against those involved in the communications endeavor.
Within any specific communications endeavor, there are both predictive and prescriptive assumptions. These assumptions are better visualized by such variables as the traits each person has (such as age or sex), the environment, and the nature of the interaction itself. Age and sex might have assumptions built into them, as does an environment, such as a job interview. The actual purpose of the interaction, such as making a good impression, also has certain assumptions built into them given the nature of the other variables. This is all simple enough: the real issue is whether violating these assumptions might help or hinder the person trying to make the impression.
The above might all seem rather commonsensical. But the theoretical pay off is that such expectations might be violated to the benefit of the violator. For example, an older man applying for a job suited for a younger person (such as a sales clerk at a record store) might violate assumptions by acting more animated, knowledgeable and humorous than what is considered "appropriate" for an older man, hence impressing the interviewer. Expectations Violations Theory, in other words, might break down barriers to advancement in such a case. It can, clearly, also work in the reverse. Breaking out of assumptive barriers can work either to the detriment or to the advancement of the actor, depending on the three variables above.