This study was designed to determine if preladenant (SCH 420814, MK-3814) can reduce drug-induced involuntary movements in participants with schizophrenia or schizoaffective disorder. Participants were to be evaluated for two 14-day treatment periods with a 3-week washout period between treatment periods. The primary outcome measure, Extrapyramidal Symptom Rating Score (ESRS), was to be evaluated frequently during the treatment periods.