Trait Negative Affect (NA) and Positive Affect (PA) are strongly associated with Neuroticism and Extraversion, respectively. Nevertheless, measures of the former tend to show substantially weaker self–other agreement—and stronger assumed similarity correlations—than scales assessing the latter. The current study separated the effects of item content versus format on agreement and assumed similarity using two different sets of Neuroticism and Extraversion measures and two different indicators of NA and PA (N = 381 newlyweds). Neuroticism and Extraversion consistently showed stronger agreement than NA and PA; in addition, however, scales with more elaborated items yielded significantly higher agreement correlations than those based on single adjectives. Conversely, the trait affect scales yielded stronger assumed similarity correlations than the personality scales; these coefficients were strongest for the adjectival measures of trait affect. Thus, our data establish a significant role for both content and format in assumed similarity and self–other agreement.