In the last decade, there has been increased recognition that traits refer not only to between-person differences but also to meaningful within-person variability across situations (i.e., whole trait theory). So far, this broader more contemporary trait conceptualization has made few inroads into assessment practices. Therefore, this study focuses on the assessment and predictive power of people’s intraindividual variability across situations. In three studies (either in student or employee samples), both test-takers’ mean trait scores and the variability of their responses across multiple written job-related situations of a situational judgment test (SJT) were assessed. Results revealed that people’s intraindividual variability (a) was related to their self-rated functional flexibility, (b) predicted performance above their mean scores, and (c) predicted their actual personality state variability over 10 days. These results open opportunities for complementing traditional selection procedures with more dynamic indices in assessment.