Love Is with You

A story about a protagonist has a protagonist, a supporting character, and a villain; the protagonist will live in a world created specifically for it, encounter challenges, setbacks, and encouragement, and then take action; the same goes for telling a story about a supporting character or a villain, the supporting character and the villain are the protagonists of the story! But why can they be replaced in the same position? The world is so big, but its bigness lies in being too small and not complex?