We evaluated the accuracy of peer ratings of roommates' personality characteristics, against roommate self-ratings, as a function of rating domain observability. Instead of the usual ratings of broad personality traits, however, our domains represented peer ratings of narrow exemplars of personality traits. Specifically, we compared roommate ratings on (a) observable trait-related behaviors with (b) unobservable trait-related attitudes or beliefs. We observed greater self-peer agreement in rating behaviors, in general, than in rating beliefs. We also observed greater tendency of raters to adopt an assumed similarity heuristic when judging their roommates' attitudes and beliefs than their behaviors. We discuss the contribution of these findings to understanding the determinants of accuracy in personality judgments and developing best practices for personality assessment. © 2014 Elsevier Inc.