When survey respondents answer survey questions, they can also produce “paradata” (Couper 2000, 2008): behavioral evidence about their response process. The study reported here demonstrates that two kinds of respondent paradata – fluency of speech and gaze direction during answers – identify answers that are likely to be problematic, as measured by changes in answers during the interview or afterward on a post-interview questionnaire. Answers with disfluencies were less reliable both face to face and on the telephone than fluent answers, and particularly diagnostic of unreliability face to face. Interviewers’ responsivity can affect both the prevalence and potential diagnosticity of paradata: both disfluent speech and gaze aversion were more frequent and diagnostic in conversational interviews, where interviewers could provide clarification if respondents requested it or the interviewer judged it was needed, than in strictly standardized interviews where clarification was not provided even if the respondent asked for it.