Music is artistic and there are always other ways to do music.
A good way to think of music is as a cooperative construction project. These should also be adjusted whenever there is a huge change in the music, such as a new melody, or the change over between the vocal and instrument parts. Some times, you want to make these changes yourself alone, other times, you want to team up with another player and make these changes together or in a complementary fashion. Often, the whole ensemble should make these changes together. The instrumentalist will play many slow, long, notes that are sometimes out of rhythm or staggered compared to the basic rhythm and measures.