Co-creation with consumers has emerged as a strategic tool for companies to foster marketing innovation in advertising due to many successful campaigns. This experimental study explores the impact of two types of Verbal Personification (physical vs. mental) used in the creative brief on the quantitative and qualitative creative performance of consumers involved in crowdsourcing, considering individual differences in mental imagery ability and the mediating role of mental transportation. Confirming our assumptions, the results show that physical (vs. mental) Verbal Personification directly affects the quantitative creative performance (fluency), suggesting that physical Verbal Personification can trigger more creative ideas. This quantitative creativity also positively impacts the qualitative creativity (idea originality and attractiveness). Moreover, we show that individuals with high (vs. low) mental imagery ability provide more (vs. less) creative ideas when presented with physical Verbal Personification. However, mental transportation does not mediate the effect of Verbal Personification types on fluency as assumed. This research contributes to the literature on consumer co-creation, showing the importance of Verbal Personification type (physical vs. mental) and individual cognitive differences in stimulating creative performance. It also highlights that, in functional-oriented stimuli such as creative briefs, Verbal Personification, even a physical one, is not enough to trigger transportation. Through an experiment, this study shows that a creative brief using physical Verbal Personification—such as describing a bike as “running” and “dancing”—increases the individual quantitative creative performance (the number of ideas) of crowdsourcing participants involved in co-creation advertising campaigns. Specifically, a physically personified brief outperforms a brief using mental personification, such as describing the bike as “thinking” and “knowing,” and also a brief without any personification, in stimulating individual idea generation from these ordinary individuals. Moreover, via the quantity of ideas, a physically personified brief also enhances the originality and attractiveness of the ideas (the qualitative creative performance) proposed by the participants. Furthermore, participants with higher mental imagery ability produce significantly more ideas when exposed to physical Verbal Personification in the brief. However, such personified briefs are not sufficiently narrative to allow mental transportation of the crowdsourcing participants. Therefore, companies can improve their co-creation processes by writing briefs in a physically personified way and by selecting people with high imagery capacity for their crowdsourcing contests. The study thus offers a practical framework for managers to develop communication strategies that maximize consumer creativity, necessary for innovation and market success.