“Empezar con hacer, después pensar.” Starting by doing, then thinking. In this chapter, we present the DSIS (Deep Societal Innovation for Sustainability and Human Flourishing) model for transformation to sustainability through a dialogue with a case study of the radical democratic assemblies that emerged in the wake of the 2019 protests in Chile. On the basis of the model and the case we investigate the importance of the idea of action for prefigurative politics. In the wake of Hannah Arendt’s political thinking, we argue that action is characterised by the element of natality and leads to radical new beginnings, questioning pre-determined or utopian forms of prefiguration. The phrase: “starting by doing, then thinking,” voiced by the members of the Chilean citizen assemblies, expresses this concern for action and the importance attributed to a creative and collective doing that is not usurped by pedetermined goals. To prevent the latter from happening, we show the importance of shared values and ethical directions for bringing about open-ended forms of prefiguration that orient our political being-together towards more sustainable and inclusive horizons.