Generative Artificial Intelligence (GenAI) greatly attracts the public's interest; thus, this research investigates discussions between experts and members of the public about this new technology on social media. Using computational and manual analysis of X (formerly Twitter) data, we investigated discussion topics, the roles discussants — including both experts and public — play, and the differences between experts' posts and the public's replies. Moreover, we examined the dynamics between the discussants' roles and social media engagement measures. We found that the public is not only actively contributing to the discussion of GenAI on X, but also becoming knowledge co-producers alongside experts in the sphere.