This preliminary study examined the relationship between clinicians' perceptions (i.e., speech pathologists) of children's scores on the Assertiveness scale of the Teacher-Child Rating Scale 2.1 and gains in naming and defining words following English-only or Spanish--English intervention for emergent bilinguals (EBs). Twenty-eight Spanish-speaking preschoolers were randomly assigned to participate in one of two vocabulary intervention groups: English-only (n = 14) and Spanish-English with Spanish provided through a tablet computer (n = 14). EBs were assessed in naming and definition before and after interventions. The clinicians provided perceptions of levels of assertiveness and shyness of both groups of EBs using the Assertiveness scale. A correlation analysis indicated a strong relationship between the Assertiveness scale and English word naming and definition gains in both groups (English-only and Spanish--English) and Spanish gains in the Spanish--English group. Clinicians' perceptions of shyness should be considered as a variable of interest that affects response to intervention.