Digital health behaviour change interventions may be adapted from in-person interventions, without appropriate consideration of how the digital context might differ. Drawing on the wider literature on behaviour change intervention development, this research aims to explore the digital adaptation process of health behaviour change interventions and the specific considerations for digital modes of delivery. A qualitative interview study. Interviews with 15 intervention developers/facilitators were analysed using inductive thematic analysis. Findings highlight a continuum of digitalization, where variation in technology available and human support influences considerations for digital adaptation. 'What vs how: "trying to do the impossible"' describes the balance between retaining the intervention's active ingredients while modifying for digital delivery. Through 'Trial and error', participants described an iterative process based on experience of delivery. 'Creating connection and engagement' emphasizes the importance of social support and the challenges of replicating this. Several considerations for digital adaptations are proposed including the involvement of end-users (facilitators and recipients) during adaptation, the need to understand the original intervention and new context for use, and the different motivational needs of digital intervention recipients.