Physical activity (PA) promotion is an ideal intervention target for public health because it has the potential to help individuals feel better, sleep better, and perform daily tasks more easily, in addition to providing disease prevention benefits. There is strong evidence that individual-level theory-based behavioral interventions are effective for increasing PA in adult populations but causal inference for potential specific pathways by which (i.e. how) these interventions exert total effects (i.e. direct and indirect) is often unclearly articulated -- though frequently attempted to be estimated. Thus, the three objectives of this tutorial were to demonstrate (a) common cause confounding assumptions required for identification of direct and indirect causal effects; (b) how to estimate identified direct and indirect causal effects of an individual-level theory-based PA-promoting intervention; (c) how to perform sensitivity to violation of common cause confounding assumption(s) analyses. The demonstration was based on the Well-Being and Physical Activity study (ClinicalTrials.gov, identifier: NCT03194854).