The instructional performance of international teaching assistants (ITA) in U.S. universities is generally considered as problematic due to linguistic and cultural differences in existing studies. Drawing on interactional sociolinguistics, conversation analysis, and positioning theory, this study aims to find out how ITAs are juxtaposed between content expert and language novice in the actual instructional interactions with U.S. college students. The data under study consist of a number of dyadic office hour interactions. The matter of analytic interest is the type of sentence completion that U.S. college students use to assist or improve on ITAs' instructional discourse. While interactionally accommodating, sentence completion may position the ITA instructors as language novices, and furthermore cast doubt on their situated identity as instructors. The findings here may shed some light on our understanding of how ITAs are negatively perceived and even stereotyped in their instructional discourse.