A variety of subtle social cues, including gaze behaviour, are used to form impressions of others. For example, if another’s eye-gaze reliably helps or hinders us while we complete a task, we incidentally form a positive or negative impression about them. In real life, people are rarely so consistent in their behaviour, and they are often encountered in dynamic group contexts. To date, however, it is not yet known how incidental impressions are affected by either changes in target individual’s behaviour over time group, or by the group’s behaviour. To better understand how impressions are formed when subtle social behaviours change valence over time, we manipulated helping behaviour both at the level of the individual (Experiments 1-3) and the wider group (Experiments 4 & 5). Contrary to the idea that first impressions are hard to change, we found no evidence that impressions were driven by initial behaviour (primacy effects). Rather, people tended to form impressions based on the most recent behaviour, with some influence from the overall, average behaviour. In addition, we found that individuals’ behaviours appear to be viewed more or less favourably, depending on the behaviour of the wider group. Overall, we demonstrate that impression formation based on subtle social cues is not dominated by a single process, but instead reflects a complex product of cognitive mechanisms that integrate average valence over time, the direction of behaviour changes, the recency of observed behaviour, and the group context in which the behaviour is observed.