This thesis investigates the role of secondary emotions in action selection, and how this affects the believability of a character. A mod for a First Person Shooter (FPS) game “2 minutes of Mayhem” was developed in Virtools game engine. The modification involved adding an affective component into the game. This component created secondary (complex) emotions by combining two primary emotions using fuzzy logic. An action selection mechanism was created using Finite State Machine and Nash Equilibrium. This mechanism relied on the secondary emotions as its main input. It coupled the secondary emotions together with the agent’s goals and came up with the relevant action that an NPC (Non Playing Character) should take. In order to evaluate the role of secondary emotions in action selection and its effects on believability, six tests were developed. These tests were based on sub-dividing the problem formulated into six hypotheses. A total of 60 subjects were involved in the final exploratory study. The results of the study show that secondary emotions play a crucial role in action selection, and as a result they enhance the believability of a character.