Effective police interviews are central to the justice process for sexual assault victims, but little is known about either actual communication between police officers and witnesses or the alignment between guidance and real practice. This study investigated how police officers, in formal interviews, follow 'best evidence' guidance to obtain victims' demonstrable understandings of 'truth and lies'. We conducted qualitative conversation analysis of 20 evidentiary interviews between police officers and victims who were 'vulnerable' adults or children. Analysis revealed that interviewers initiated conversation about truth and lies inappropriately in three ways: (i) by eliciting "confirmations" rather than "demonstrations" of understanding; (ii) by eliciting "multiple" demonstrations and confirmations of understanding, or (iii) by "re-introducing" 'truth and lies' conversations at incorrect points in the interview. Both (ii) and (iii) imply prior or forthcoming dishonesty on the part of the victim. In the context of encouraging victims to report sexual assault and achieve justice, the article reveals potential communicative barriers in which victims--or their evidence--may be discredited right at the start of the process.