This paper presents a study that assesses the effectiveness of Motivational Interviewing (MI) interventions on the engagement of adults over 50 years to do more physical activity. While MI's four step process may easily be adapted to online devices, modeling the MI approach based on empathy may be more challenging (as opposed to directive interviews in which the user is encouraged to adopt a favorable attitude from the outset). Three devices are compared: a non-directive MI-based website, a non-directive MIbased virtual agent and a directed MI-based one. Our results show that the non-directive virtual agent tends to be perceived as more empathetic and trustworthy than the directed one,and that it significantly raises the participants' self-efficacy to overcome barriers, and positively impacts intrinsic motivation. We discuss mentions several implications for the adaptation of MI to the context of ECA. CCS CONCEPTS • Human-centered computing → HCI design and evaluation methods; Empirical studies in HCI; Web-based interaction.