Having shared or similar attributes with others in need has been shown to increase observers' empathic response, willingness to help and prosocial behaviour. We tested whether a subtle similarity, namely of observers' and targets' self-regulatory orientation in terms of a promotion or prevention regulatory focus (i.e., interpersonal regulatory fit), would entail similar effects. Interpersonal regulatory fit and misfit was conveyed through focus- congruent or -incongruent emotional reactions which targets facing distressing situations expressed. We predicted that when observers' regulatory focus fits with targets' negative emotional reaction (i.e., promotion focus and dejection reaction or prevention focus and agitation reaction), they would be more likely to express empathy, willingness to help, and to engage in prosocial behaviour towards this target compared to conditions of misfit. Five studies examined observers' chronic and situationally induced regulatory focus (Studies 1, 3, & 4 and Studies 2 & 5 respectively) and presented different distressing scenarios with targets conveying focus-congruent or focus-incongruent negative emotions. Inconsistent results emerged across studies, which indicated misfit, fit and no effects. An internal meta-analysis across all studies indicated that overall there was no evidence of either a fit or a misfit effect. Two further studies (Study 8 & 9) including a stronger portrayal of target's regulatory focus and attempted replication of a previous study also yielded inconsistent findings. A systematic review was carried out to examine the important moderators affecting the interpersonal regulatory fit phenomenon. Lastly, we explored further divergent avenues of research, as informed by regulatory focus theory broadly, with two studies addressing social projection and empathy, as well as violation of expectations and social perception (Study 6 & 7). Overall, this work sheds light on the technical challenges when exploring relations between subtle interpersonal regulatory (mis)fit and prosocial reactions. Implications for future research are discussed, including the importance of creating stronger interpersonal (mis)fit experiences by exploring other mediums of presentation and emphasising distressed targets' hindered goal pursuits in addition to negative emotional reactions.