Previous theories on expert involvement have been classically based on the assumption of extrinsic influence-driven motivation. This article aims to construct an expert behavior theory to bridge the gap between intrinsic motivation and behavior of experts by identifying two dimensions, namely, the attitudes toward two relationships, between theory and practice and between experts and officials. These two intrinsic motivations determine the four roles of individual experts, namely, technology communicators, theory demonstrators, idea entrepreneurs, and knowledge brokers. The empirical strategy is based on a comparative study of four experts who were involved in the Wenling participatory budgeting reform in China.