This paper describes the context in which a theoretically driven physical activity counselling intervention was developed as a pragmatic treatment for primary care patients with depression. The intervention, delivered by Physical Activity Facilitators (PAFs), alongside usual care, was developed and piloted, then compared with usual care within a randomised TRial of Exercise and Depression in the UK (TREAD-UK). Details about the background of the intervention are provided, including underpinning principles, theoretical framework (i.e., Self-Determination Theory; Deci & Ryan, 2000) for behaviour change, counselling style and techniques (drawn mainly from motivational interviewing), contextual issues, and delivery issues associated with the PAF. In particular, we provide an outline of the content of a training programme for the PAFs. A detailed description of how we operationalised the intervention including aims, content and techniques, and strategies employed by the PAFs is provided. Finally, we reflect on future implementation issues.