Effective interaction across organisational boundaries is a critical success factor in new product development (NPD). However, few studies have investigated how different mechanisms enable effective interaction across organisational and particularly hierarchical boundaries. This study explores how the formality of the NPD process influences the nature of interactions across different organisational boundaries and specifically identifies interaction mechanisms used across hierarchical boundaries. Cross-sectional interviews were conducted in nine firms. Findings highlight that in firms with a formalised NPD process, interactions tend to have a transactional/managerial bias. In contrast, in firms where the NPD process is flexible, interactions have a more social objective.