Inner speaking, the production, and experience of verbal language without any audible vocalization is a critical component of inner experience and imagination. The role that (un)certainty plays in idea generation might explain the unique ways in which idea generation is characterized and affected by different types of inner speech. To explore this open problem, an experiment with a within-subject design (n = 202) was conducted. The results suggested that certainty about the potential of selected information for generating original and useful ideas, elicited using creativity instructions, caused increased self-reinforcement, self-management, and simulation of social interactions with imagined others by inner speaking, but did influence self-critical inner speaking. Self-reinforcing inner speaking, and possibly the simulation of social interactions, subsequently affected the degree of originality and usefulness participants attributed to their ideas. Herewith, the present study contributes novel insight into how inner speaking characterizes and affects idea generation.