It is well established that interviewers learn behaviors both during training and on the job. How this learning occurs has received surprisingly little empirical attention: Is it driven by the interviewer herself or by the respondents she interviews? There are two competing hypotheses about what happens during field data collection: (1) interviewers learn behaviors from their previous interviews, and thus change their behavior in reaction to the behaviors previously encountered; and (2) interviewers encounter different types of and, especially, less cooperative respondents (i.e., nonresponse propensity affecting the measurement error situation), leading to changes in interview behaviors over the course of the field period. We refer to these hypotheses as the experience and response propensity hypotheses, respectively. This paper examines the relationship between proxy indicators for the experience and response propensity hypotheses on interview length using data and paradata from two telephone surveys.Our results indicate that both interviewer-driven experience and respondent-driven response propensity are associated with the length of interview. While general interviewing experience is nonsignificant, within-study experience decreases interview length significantly, even when accounting for changes in sample composition. Interviewers with higher cooperation rates have significantly shorter interviews in study one; however, this effect is mediated by the number of words spoken by the interviewer. We find that older respondents and male respondents have longer interviews despite controlling for the number of words spoken, as do respondents who complete the survey at first contact. Not surprisingly, interviews are significantly longer the more words interviewers and respondents speak.