When and how to survey potential respondents is often determined by budgetary and external constraints, but choice of survey modality may have enormous implications for data quality. Different survey modalities may be differentially susceptible to measurement error attributable to interviewer assignment, known as interviewer effects. In this paper, we leverage highly similar surveys, one conducted face-to-face (FTF) and the other via phone, to examine variation in interviewer effects across survey modality and question type. We find that while there are no cross-modality differences for simple questions, interviewer effects are markedly higher for sensitive questions asked over the phone. These findings are likely explained by the enhanced ability of in-person interviewers to foster rapport and engagement with respondents. We conclude with a thought experiment that illustrates the potential implications for power calculations, namely, that using FTF data to inform phone surveys may substantially underestimate the necessary sample size for sensitive questions.