Even though numerous factors are likely to contribute to the outcomes of both naturalistic and instructed second language acquisition, such as the amount of exposure to the target language (TL), the beliefs, dedication, experience, involvement and qualifications of the instructor, or the teaching methods and materials employed, individual learner differences appear to play a significant role in second or foreign language (L2) learning. While specific classifications vary, such individual difference (ID) variables can be sociocultural (e.g., beliefs, attitudes, experience), cognitive (e.g., age, aptitude, working memory, cognitive styles, learning strategies), and affective (e.g., anxiety, personality, motivation, willingness to communicate) in nature, with some IDs cutting across clear-cut categories, themselves being affected by each other and a host of other variables (Dörnyei & Ryan, 2015; Gregersen & MacIntyre, 2014; Pawlak, 2012a). This paper provides a brief overview of selected factors in these areas, with particular emphasis being placed on the influence that they are likely to exert on interactions that take place between learners, and between learners and instructors in the language classroom, as well as the ways in which such interactions can contribute to L2 learning. With this in mind, some key assumptions will first be spelled out, which will be followed by the discussion of the latest research findings in each area, subsequently focusing on research undertaken in the interactionist approach. Finally, future research directions will be outlined, methodological issues will be touched upon, and implications for classroom practice will be considered.