The aim of this study is to explore the factors that motivate the choice between active (Experiencer-Object) and middle (Experiencer-Subject) constructions with Psych-II verbs in Spanish, for example "A ella no la/le asustaba el contagio" vs "Ella no se asustaba de nada". The analysis pays particular attention to the choice between accusative and dative case for the object in the active clauses, to determine how case alternation and diathesis alternation are related. Using data from ADESSE database (265 psych verbs and 5,845 clauses) and CORPES XXI (50 psych verbs and 169,338 clauses), we performed a regression analysis and found that the Experiencer Person, Aspect, and Verb are the main significant factors for the use of the Experiencer-Object vs Experiencer-Subject pattern. Besides, significant predictors for the selection of Accusative vs Dative case of the Experiencer-Object are Zone, Stimulus Person, Aspect and Verb. However, notwithstanding some parallelisms in the predictors, we do not find a significant correlation between diathesis alternation (Subject vs Object Experiencer) and case alternation (Accusative vs Dative Object Experiencer) with verbs that allow for both kinds of alternation. This leads to the conclusion that each construction type has different communicative functions.