What is the nature of a social interaction? The subjects interacting have their own nature, but what about the interaction itself? It will no doubt reflect the nature of the interacting subjects, but it may be expected to have a nature of its own. There have been varying views of this, more or less explicit, in social psychology and elsewhere, which I will list. The simplest view might be called causal: the idea would be that individual subjects causally interact in the manner of physical bodies, so that there is nothing distinctive about human social interaction—it’s all just pushes and pulls. A second view brings in the concept of information: in a social interaction information is exchanged, linguistically and otherwise, so that the informational state of each subject is altered. A third view might be called epistemic: each subject has a certain kind of knowledge of the other, typically mutual knowledge, as in “x knows that y knows that x knows etc.”. Fourth, there is the interpretative view: each subject is engaged in interpreting the other’s behavior, linguistic and non-linguistic, either radically or domestically, using psychological and semantic concepts. Fifth, the relation is one of influence: one subject influences another in certain ways, say by inducing conformity. Sixth, the idea of power is invoked: subjects exercise power over other subjects, sometimes detrimentally, this being the essence of social interaction. Seventh, the notion of competition is added: individuals compete with each other, biologically or economically or in some other way, the result varying from death to status superiority. Eighth, cooperation is taken to be the essence of social interaction: people bond and band together to achieve a joint goal, as in hunting parties, political parties, and party parties. Ninth, we have the existentialist view of social interaction: individual subjects of consciousness confront each other in their radical freedom, sometimes in bad faith, possibly in a condition of authenticity. Tenth, there is the theatrical conception: the nature of social interaction is inherently dramatic or histrionic, as each party plays a part on the social stage. Eleventh, we have what might be called the spiritual theory: social interaction is a meeting of souls, an escape from spiritual solitude, and a glimpse into the inner spirit of the other. Finally, there is the moral conception: social interaction is the exercise of moral duties, an expression of virtues and vices, the place where our moral nature shows itself.
I take it these positions will seem familiar, with familiar names attaching: Shakespeare, Goffman, Rousseau, Marx, Sartre, Foucault, Quine, Davidson, Darwin, Kant, Hume, and many more. Clearly, not all of these views are in competition with each other, except perhaps as declarations of emphasis; but we can see that different concerns, practical and theoretical, lead to different conceptions of the nature of the social. One might adopt an eclectic position: all the views mentioned have an element of truth to them, social interaction being a multi-dimensional affair. But one might hope for something a bit more organized and systematic; one might seek to identify the kernel of social interaction, from which other features flow. So I will suggest a conception that seems to me to capture the basic character of the social: it combines the epistemic, the existential, and the theatrical conceptions. In a social interaction we possess a certain kind of mutual knowledge, as we recognize the way the other is forming a view of our views of him or her; and so on indefinitely. I know that you know that I know that you can’t be trusted, say. This doesn’t apply to social interactions involving animals or small children, but it is the bane of adult human social interactions: we are always wondering what the other thinks of us, especially with regard to what we think of them. The existentialist component comes in via free action: we see the other as a free agent who may or may not act as we would wish. We are aware that the person we are interacting with may not act as we desire—that she might, say, suddenly decide to leave, or to end the relationship. This awareness of freedom hovers over all our social interactions: they are nothing like our interactions with inanimate objects. Anxiety is the natural outcome, so the social encounter is always fraught, always tenuous. The theatrical component concerns our methods for handling social interactions: we have to construct a social self (or selves) that we present to others for their consideration and treatment. We must play a part that will oil the encounter, produce the requisite attitudes in others, and manage the freedom that we know hovers ominously in the background. Thus our social interactions combine a special type of mutual knowledge, an understanding of freedom, and theatrical skill. That is their basic structure, from which other features emanate. And it is what might be lacking in abnormal cases—a certain package of cognitive competences. This could come about by way of upbringing or innate endowment, resulting in social incompetence or worse. And it is not as if this stuff is easy, some people being much better at it than others. It is a developed skill, a specific mental module, with a rich internal structure. It grows in the child and can atrophy in the adult. Disease can disrupt it. Practice helps. It needs nurturing.
There should really be a philosophy of social interaction—a systematic attempt to delineate its structure. It would identify the basic components and seek to derive the social phenomena we observe in various contexts—economic, marital, athletic, musical, familial, pedagogical, etc. Philosophers have analyzed the human subject and human society, but they have not developed a branch of study dedicated to the nature of social interaction as such, except as this might serve some other agenda. Social psychology, too, might benefit from this kind of philosophical attention. It never hurts to be clear about what you are talking about. 
 When I studied social psychology in the late 1960s there was much concern with issues of obedience and conformity, possibly as a consequence of prevailing social conditions. But I don’t recall any preliminary discussion of the general nature of a social interaction, so the subject matter was left vague and indeterminate. If I were teaching social psychology today, I would spend some time exploring different conceptions of social interaction: what is it to interact socially with another person? And I would frame the issue as concerning a certain mental competence, Chomsky-style. What is this competence, how is it acquired, how is it expressed, what are its pathologies, how might it be improved? The philosopher might be useful here.