--- - |2+ Abstract Objective Studies on situation and Person × Situation interaction effects often use only one method of measurement, so that the extent to which the effects may be method‐specific cannot be determined. We introduce a new multimethod latent state–trait model for random and fixed situations (MM‐LST‐RF), which allows examining person, situation, and Person × Situation interaction effects in the context of multimethod measurement designs (e.g., studies with multiple reporters), and discuss potential areas of application for the new model in personality research. Method The new model allows analyzing novel features of multimethod, multi‐situation data, including (a) the convergent validity and method specificity of trait (person) effects, situation effects, and Person × Situation interaction effects; (b) the degree of situation specificity of method effects; and (c) potential Method × Situation interactions. An application to smoker’s affect (N = 235; 57% female; 93% Caucasian) before and after quitting smoking is presented with positively and negatively worded items as methods. Results The MM‐LST‐RF model fit the smoking data well. Method specificity of many effects was high. Conclusions The MM‐LST‐RF model provides researchers with a new framework for testing method specificity of person, situation, and interaction effects. - Journal of Personality, EarlyView.