Dynamic assessment has been proven to effectively promote EFL learners' speaking proficiency, but its implementation in teaching speaking skills has been limited. One of the main reasons is that, thus far, very few studies have scrutinized the impacts of its two main models, interactionist and interventionist, on the speaking sub-skills of EFL learners. Instead of examining speaking as a general skill, this research focused on four speaking sub-skills, including grammatical range and accuracy, vocabulary, fluency, and pronunciation, with a concurrent experimental mixed-method design. To this aim, 30 undergraduate students from the Islamic Azad University, North Tehran Branch, were recruited through convenience sampling and participated in both phases. The quantitative data were collected from participants' pre-and post-test scores in the IELTS speaking module, and qualitative data consisted of the transcribed recordings of the intervention sessions. The integration of the quantitative and qualitative results revealed that while both models positively affected EFL learners' speaking proficiency, they differed in the extent and mechanisms of their impacts. The interactionist DA effectively improved grammatical range and accuracy, pronunciation, and vocabulary depth, and the interventionist DA enhanced fluency and vocabulary breadth more efficiently. The findings dissect the differential effects of DA models in one-to-one speaking lessons, offering practical insights for ELT practitioners and stakeholders.