Facial behaviour is a central component of social interaction, yet is primarily studied in the absence of a social partner with a heavy focus on static, universal prototypes. Examination of individual variation in facial behaviour embedded within social interaction is essential to understand social function. We conducted a three-part multi-method study to examine how variation in facial behaviour impacts first impression formation, social outcomes during social interaction and perception from third-party observers. We recorded a semi-structured social interaction with fifty-two participants which involved interacting with a confederate via video call across various social contexts. A facial expressivity index was extracted using FACS (facial action coding system), and personality was measured by self-report. A subset of these participants (n=34) also completed tasks in which they attempted to achieve social goals via facial communication. Clips from the video call and the social tasks were then rated by an independent sample of 176 participants. Facial expressivity varied considerably across participants, but little across behavioural contexts. More facially expressive participants were more agreeable, more well-liked, and more successful at negotiating (if they were also more agreeable). Participants who were more facially competent, readable, and perceived as readable were also more well liked. Our findings suggest that facial behaviour could be a stable individual difference, and that it proffers social advantages which may vary depending on personality. These results point towards an affiliative function of facial behaviour.