Please bear with me as I try to explain what I'm looking for; my question concerns getting help to find the right words to describe it. Also, if there's a more appropriate Stack Exchange site for a question like this, please feel free to redirect me in a comment.

I am looking for a generic and appropriate term in social science theory for a particular meaning of "manager" in business management theory. The particular meaning I use here is this: **a "manager" is someone with the ability to willfully change one or more variables in a theoretical model**. Here are a few examples:

* Theory S: sales are affected by store location, advertising and weather. A ***manager*** in this case can freely change the amount of advertising. They might or might not be able to change the store location. Although they have no influence over the weather, they can make decisions (such as to advertise outdoors or not) in response to the weather.
* Theory ES: employee satisfaction depends on stress levels of the job, salary, and daily commute time. A ***manager*** (of the employee) might be able to freely change the salary, has some control on the stress levels, but has limited control over commute times (if telecommuting would be infeasible).

The key point in these examples is that a "manager" has some control over some of the independent variables of the theoretical models, and so can influence the outcome dependent variable in a desirable direction.

Let's go deeper. In my desired meaning of "manager", in Theory ES, the employee is also a "manager" concerning that theory: they have some control over their own job stress levels (by changing their work habits) and their own commute times (by changing their mode of transportation, or by moving their home). So, they, too, have some control over the outcomes.

I am looking for an appropriate technical term that captures the role of such a person who can willfully change the values of one or more of the independent variables so to influence the dependent variable outcomes in their favour. While "manager" works as a term in the case of Theory S, it obviously doesn't work well in the case of Theory ES since the employee is also a "manager" in that sense. Thus, I am seeking a more general term that captures this meaning.

But my application of this term goes far beyond business management. I want a term that is ideally meaningful in any theoretical context, especially in the social sciences, but hopefully even in any empirical science. Here are other examples of "manager" in other disciplines (sorry for the poor examples; I'm not an expert in any of these fields, so I will have to resort to stereotypes to make my point):

* Psychology: a parent could be a "manager" in effecting their children's self esteem (DV) by changing their parenting approach
* Sociology: a group member could be a "manager" in affecting group performance outcomes by choosing whether to willingly cooperate with or willingly frustrate other members of the group.
  * I think perhaps the term for "manager" in this context might be "social actor".
* Physics: when an experiment is carried out, the scientist conducting the experiment is the "manager" since they purposely alter parameters in order to change outcomes.
* Geology: in studying, for example, the shape of cliffs from interaction of sea waves with wind patterns over a ten-year period, there is no "manager" in this case, since the outcomes and variables of interest have no human will involved. (Even if human-caused climate change were implicated, there is no "manager" since there is no human consciously taking directed willful action with a specific intention in this case.) 
* Medicine: if a patient influences their cancer outcomes by their choice of frequency and type of tobacco that they consume, the patient is a "manager" in influencing their cancer outcome.

So, **I am looking for a more suitable term that someone captures my broad meaning of "manager" here**. "Social actor" from sociology seems close, but I'm not sure if that quite captures it.

**EDIT:**

One answer suggested "agent". I like the generic idea of "agent". However, although I already thought of that as a possibility, I hesitate on it because in business management (my field), "agent" and "agency" have a very particular widespread meaning--an agent is someone who is not an owner (the principal) but who acts on behalf of the principal. In that context, the term "agent" is usually employed when exploring conflicts of interest when the agent's best interest does not align with that of the principal. So, I would prefer a term more generic than "agent" if one exists.