Project-specific actor constellations and relationships shape contemporary planning practice. The existing literature predominantly focuses on the flexibility of regulations in planning projects and largely ignores the behaviour of involved governance actors. Here, we argue that the nuances of relationship dynamics between public and private sector actors deserve increased scrutiny. Based on in-depth interviews with public planners and project managers, and private developers and investors involved in Amsterdam's urban development, and a case study of a major redevelopment project, we have coined a new term: 'behavioural flexibility'. Behavioural flexibility highlights how relationships extensively affect how actors trust and communicate with one another, and how their goals align. Combined, these factors strongly impact planning outcomes as they determine how actors eventually behave in projects by either being supportive and constructive or unhelpful and obstructive. The findings call for an assessment of 'indicative actor relationships' as an alternate starting point for planning projects.