Researchers agree that informal professional learning is crucial for developing new EAP teachers' expertise, but there exist few teacher-centred accounts of how context-specific projects and practices create favourable conditions for such learning. Through presenting and analysing a collaborative action research (CAR) project, this paper illustrates how CAR can facilitate informal learning among new EAP teachers. The project was conducted in the context of a large-scale EAP course at a Singaporean university, where three teachers, each possessing under three years' experience teaching EAP, co-created an 11-page analytic rubric and reflected on their collaboration. Drawing on the written reflections of these three teachers, this paper examines how collaborative rubric development created a "backstage" setting where teachers engaged in trusting and stimulating dialogue, aimed at addressing shared concerns surrounding performance expectations and scoring practices. Through the process of drafting, revising, disseminating, and using the rubric, the three teachers clarified and adjusted their own assessment-related beliefs and practices, reflected on the limits of using rubrics to align assessment standards, and experimented with different methods of sharing their findings with other teachers. In the process, the rubric was reinvented as a dialogical tool that enabled teachers to scaffold professional learning for each other.