Functional Analytic Psychotherapy (FAP) is a form of therapy grounded in behavioral principles that utilizes therapist reactions to shape target behavior. Despite a growing literature base, there is a paucity of research to establish the efficacy of FAP. As a general approach to psychotherapy, and how the therapeutic relationship produces change, FAP does not specify a disorder construct as a dependent variable. In addition, because of its functional, idiographic nature FAP is also challenging to specify as an independent variable. The present paper suggests interpersonal relating as a domain of focus for a line of FAP research and describes four levels of FAP application to contribute to specifying FAP as an independent variable for use in research. Illustrative data from an ongoing study comparing FAP to supportive listening for interpersonal difficulties demonstrates an application of these suggestions for promoting FAP efficacy research. (Contains 1 figure.)