This thesis examines what game designs cause players to exhibit communication, cooperation and prosocial behavior towards each other in cooperative action games. This study used qualitative research methods to analyze a cooperative action game called Warhammer: Vermintide 2 as a case study. The gathering of data is done through Stimulated recall. The data gathered from the analysis provides information about their behaviors and how players communicate with each other. The results indicate that players' communication, cooperation, and prosocial behavior increased during threatening situations and when aid is required by the players. Players' expectations of others changed depending on their level of experience in the game, and on the console.