Rowling usually maintains pretty tight control over her work

I've heard conflicting stories on this. Every author I know gives up all creative control when they license their work. This is required by the studios, they don't make deals with authors who try to retain control. They may make a separate deal for the author to write the screenplay but even this doesn't give control over casting or other production.

Good studios will try to work with the author if they are helping the project. The author can often prove useful in visualizing characters, scenes, settings, etc. where the text is lacking. Rowling is said to have demanded the films include all British cast, to what extent the studio was obligated to listen or simply agreed with her idea is unclear. Rowling is rare in that her name was a marketing asset so having her directly involved in the project was seen as a benefit.