As an alternative to conventional instruction and evaluation methods, dynamic assessment aims to promote language learning by utilising an interactive approach. As a subset of dynamic assessment, the interventionist approach to dynamic assessment focuses on mediation from implicit to explicit. In spite of its central role in language learning and assessment, the interventionist dynamic assessment has received less attention in empirical research. To fill part of the gap, this study examined the contributions of interventionist dynamic assessment to the acquisition of modal verbs treating working memory and field (in)dependence as two moderating variables. Two intact classes of experimental and control groups, each with 30 subjects, took part in this quantitative quasi-experimental research. The experimental group received an interventionist dynamic assessment based on the target linguistic feature, while the control group received a traditional presentation-practice-production mode of instruction. The findings of the study revealed that while at the onset, both groups performed similarly, the experimental group outperformed the control group on the posttest, as determined by the between-subjects ANOVA. The difference between pretest and delayed posttest scores was also significant, corroborating the effectiveness of the treatment on the acquisition of the linguistic form. The study findings highlighted how interventionist dynamic assessment along with a high working memory and a field-independence trait influence the acquisition of linguistic features. The study has multiple implications for EFL learners and teachers.