Bapereih is a traditional music competition among gandang tasa groups (traditional music ensembles) within the cultural region of Pariaman, West Sumatra, Indonesia. The judging system in the bapereih competition is unique, as it considers mistakes made by players from both musical and non-musical aspects, assigning them negative points. The smallest accumulation of errors determines the winner. Calculating negative points creates psychological pressure on players, making them afraid to make mistakes, thus affecting their expression and playing behavior, leading to tension, stiffness, lack of fluidity, and gimmicky strokes to avoid errors. This research aims to uncover the musical expression and behavior of players in bapereih performances. The research method employed is qualitative with a case study approach. Data collection involves carefully observing players' expressions and behaviors in detail to understand their musical expressions and playing behavior. The research results indicate that psychologically, the pressure from the competition affects the players' mentality and expression, making them stiff, tense, and less fluid.