In Baldur's Gate, if an NPC is absent when the game calls for them to recite lines, Biff will appear say their lines instead, being the understudy. Though he will say their dialogue, he will not perform any of their actions. Should you happen to kill him, there will be no penalty.
In Baldur's Gate II, Biff has an actual role to play. In the bottom floor Five Flagons Inn there is a play going on. One of the actors, Haer'Dalis, is missing so Biff comes on the scene and fills in his role. Because he isn't the usual actor in the plays in the inn and because he gets most of his lines wrong, he is booed by the crowd. Due to this the play is abandoned.