Expanding Individual Difference Research in the Interaction Approach: Investigating learners, instructors, and other interlocutors demonstrates why investigating the individual differences of all interlocutors with whom learners interact – including peer and heritage learners, instructors, researchers, and native speakers – is critical to understanding how second and foreign languages are taught and learned. Through state-of-the-art syntheses detailing what is known about learners and instructors, and novel empirical studies highlighting new avenues of inquiry, the volume articulates the most pressing needs for individual difference research. The book concludes with a scoping review, which reveals the many interlocutors still yet to be empirically considered and outlines next steps for this research. Uniquely combining linguistic theory, research synthesis, and empirical study, this book encourages students and established scholars alike to expand their conceptualization of individual differences. By demonstrating the importance of considering the individual differences of all interlocutors, the studies are also highly relevant to those teaching second and foreign languages in diverse contexts.