Expectancy effects are crucial determinants of placebo responses; however, neglecting differences in agent-specific beliefs may lead to an underestimation of response effects. This study had two main goals: (1) to theoretically explore how individual differences in expectations about agents influence response outcomes, and (2) to create a visual tool for assessing stimulus-specific expectancies. The sample included 435 adults (58.5% female) who answered demographic questions and completed six image-based items (red pill, white powder, green drink, chocolate bar, injection syringe, and electrical stimulation device), each designed to assess beliefs about the potential of these agents to boost performance. Expectancy assessments focused on anticipated physical and mental performance effects rather than on clinical or therapeutic outcomes. Logistic modeling illustrated that combining high- and low-expectancy individuals can attenuate apparent group-level expectancy effects, highlighting a potential measurement and inference concern in placebo research. Participants who were non-superstitious and non-religious reported significantly lower stimulus expectancy than others. Confirmatory factor analysis (CFA) confirmed the unidimensional structure of the Stimulus Expectancy Assessment Tool (SEAT), showing excellent fit (CFI = 0.974, TLI = 0.952, RMSEA = 0.086, SRMR = 0.032) and good internal consistency (Cronbach's α = 0.84). The latent factor accounted for an average of 48% of the variance in expectations, ranging from 21% to 66% across the six indicators. These results suggest that placebo effects may be underestimated when variability in expectancy is ignored. The SEAT offers a flexible, image-based method for identifying stimulus-specific expectancy profiles.