  Delivery of digital behaviour change interventions which encourage physical
activity has been tried in many forms. Most often interventions are delivered
as text notifications, but these do not promote interaction. Advances in
conversational AI have improved natural language understanding and generation,
allowing AI chatbots to provide an engaging experience with the user. For this
reason, chatbots have recently been seen in healthcare delivering digital
interventions through free text or choice selection. In this work, we explore
the use of voice-based AI chatbots as a novel mode of intervention delivery,
specifically targeting older adults to encourage physical activity. We
co-created "FitChat", an AI chatbot, with older adults and we evaluate the
first prototype using Think Aloud Sessions. Our thematic evaluation suggests
that older adults prefer voice-based chat over text notifications or free text
entry and that voice is a powerful mode for encouraging motivation.
