The objective of this research is to better understand experts' contributions to the prospective evaluation of a new audit standard—in this case, key audit matter (KAM) reporting. To this end, we assisted the Canadian Auditing and Assurance Standards Board by leading its consultation of 22 expert financial statement users. The methodology employed to observe our participants' opinions and cognitive processes involves thought protocol and interviews. By analyzing the rhetorical base of experts' prospective analysis, we show that our participants' arguments are often laden with postulates and lack data points, leading to generalizations. Sounder arguments entail more nuanced views but lead to uncertainties. We therefore highlight a tension between the rhetorical content of experts' insights and the calculative rationality of a cost‐benefit analysis. We also find that experts with less cognitive flexibility are less likely to be supportive of the adoption of a standard implying a change of habits in the way they process information. This tension and cognitive bias generate a significant interpretive challenge to determine a clear and dominant stance in the consultation. We discuss the implications of these findings for the legitimacy of prospective evaluations and the conduct of cost‐benefit consultations with experts. We also contribute to the literature on KAMs by substantiating concerns about the value of extended auditor reports to users.