Movie studios and production companies RARELY allow the original rights holders any concessions. They want full control. So "generally speaking" is almost as good as saying "always." And no, at least as far as the first 4 films, JK Rowling had absolutely no real authority. By the time the first film was made, the series was massively popular, but was only on the third book. They deferred to her quite a bit, but were under no contractual obligation. Her only stipulations were that each book have a separate movie and the cast be British.