Purpose In some cases of wrongful convictions, demeanour seen as inappropriate can trigger suspicions of guilt. Two experiments systematically manipulated the demeanour of criminal suspects in interrogations to test its impact on guilt ratings. Methods In Experiment 1 (N = 60), participants saw a videotaped interrogation in which the suspect displayed flat demeanour or emotional demeanour. Before viewing the interrogation, participants were told that normal reactions to trauma consisted of either flat or emotional demeanour. In Experiment 2 (N = 147), the presence of the suspect's coerced confession and demeanour evidence were both manipulated. Results In Experiment 1, a suspect who displayed flat demeanour during the interrogation produced higher ratings of guilt than did a suspect who displayed emotional demeanour, especially when participants were told to expect emotional demeanour. In Experiment 2, without a confession, flat demeanour inflated guilt ratings, whereas emotional demeanour slightly (but non‐significantly) decreased guilt ratings compared with a no demeanour information condition. When a confession was introduced, guilt ratings increased for all groups, with the highest ratings in the emotional demeanour condition. Conclusions Flat demeanour biases judgments against defendants. On its own, emotional demeanour is neutral (or potentially exonerating), but when paired with a confession, it becomes just as incriminating as flat demeanour. Recommendations for educating police professionals on the wide range of appropriate reactions to trauma are described.