Card offers three guidelines for choosing a viewpoint character who isn't the protagonist, such as in The Great Gatsby or Wild Seed:
- The viewpoint character must be present at the main events.
- The viewpoint character must be actively involved in those events, not always a chance witness (such as Watson from Sherlock Holmes).
- The viewpoint character must have a personal stake in the outcome, even though the outcome depends on the main character's choices.
Card also has a book entitled Character and Viewpoint, which dives deeper into this topic.
What about you? Have you even written something where the viewpoint character and the protagonist were separate? I never have, but it's a challenge worth considering!