To discover what role to play in the music, work with the musical director, composer or arranger and use your musical intuition. Ask yourself what the music needs. Is it grooving, or more coloristic? Is it mysterious, or driving? (Or mysteriously driving..) What is the story the arrangement is telling? What are the dynamics in each section, and when does it peak? Listen to the sum total of all the instruments (and vocals) and find your spot.