How Effective Is Author Input In Book-To-Movie Adaptations?

How Effective Is Author Input In Book-To-Movie Adaptations?

The Harry Potter movie franchise. The Hunger Games film trilogy. The Bridgerton Netflix series. The Twilight Saga movie series. The Fifty Shades of Grey movie trilogy. These book-to-movie adaptations all have one thing in common: the active influence of the authors in the adaptations. But what are the drawbacks of having the authors so closely involved?