Our research investigated whether the presence of an interface agent - or virtual assistant (VA) - in a self-service checkout context has behavioural effects on the transaction process during particular tasks. While many participants claimed to have not noticed a VA within the self-service interface, behaviour was still affected, i.e. fewer people made errors with the VA present than in the voice-only and control conditions. The results are explained as reflective of an unconscious observation of non-verbal cues exhibited by the VA. The results are discussed in relation to possible behavioural outcomes of VA presence.