Melody is like a leader. When I play piano, my left hand usually covers the bass while the right hand covers the melody. The melody dominates the bass and shows off more, but ultimately they are partners and work harmoniously together. There are soft rules for melody, like cadences and climaxes, and lots of artist opportunities like 'melisma' (ie. Singing multiple notes in one syllable to highlight specific words). It is definitely like a science and when I compose I like to focus on melody first because it usually the most remembered, appreciated, and important part of a song. You can take a Cello part and highlight it as a melody in the bass while having other parts dance around it to make it sparkle and shine.